diff --git a/homeassistant/components/data_grand_lyon/config_flow.py b/homeassistant/components/data_grand_lyon/config_flow.py
index 757e36cbe3f9..7bcabe8656d6 100644
--- a/homeassistant/components/data_grand_lyon/config_flow.py
+++ b/homeassistant/components/data_grand_lyon/config_flow.py
@@ -30,6 +30,12 @@ STEP_USER_DATA_SCHEMA = vol.Schema(
     }
 )
 
+STEP_RECONFIGURE_SCHEMA = vol.Schema(
+    {
+        vol.Required(CONF_PASSWORD): str,
+    }
+)
+
 STEP_STOP_DATA_SCHEMA = vol.Schema(
     {
         vol.Required(CONF_LINE): str,
@@ -103,6 +109,34 @@ class DataGrandLyonConfigFlow(ConfigFlow, domain=DOMAIN):
             errors=errors,
         )
 
+    async def async_step_reconfigure(
+        self, user_input: dict[str, Any] | None = None
+    ) -> ConfigFlowResult:
+        """Handle reconfiguration of credentials."""
+        errors: dict[str, str] = {}
+        reconfigure_entry = self._get_reconfigure_entry()
+
+        if user_input is not None:
+            creds = {
+                CONF_USERNAME: reconfigure_entry.data.get(CONF_USERNAME),
+                CONF_PASSWORD: user_input[CONF_PASSWORD],
+            }
+            if error := await self._test_connection(creds):
+                errors["base"] = error
+            else:
+                return self.async_update_reload_and_abort(
+                    reconfigure_entry, data_updates=user_input
+                )
+
+        return self.async_show_form(
+            step_id="reconfigure",
+            data_schema=self.add_suggested_values_to_schema(
+                STEP_RECONFIGURE_SCHEMA,
+                user_input or reconfigure_entry.data,
+            ),
+            errors=errors,
+        )
+
     async def _test_connection(self, user_input: dict[str, Any]) -> str | None:
         """Test connectivity by making a dummy API call.
 
diff --git a/homeassistant/components/data_grand_lyon/quality_scale.yaml b/homeassistant/components/data_grand_lyon/quality_scale.yaml
index fa9357574073..45d66ff0a036 100644
--- a/homeassistant/components/data_grand_lyon/quality_scale.yaml
+++ b/homeassistant/components/data_grand_lyon/quality_scale.yaml
@@ -62,7 +62,7 @@ rules:
   entity-translations: done
   exception-translations: done
   icon-translations: done
-  reconfiguration-flow: todo
+  reconfiguration-flow: done
   repair-issues:
     status: exempt
     comment: no known use cases for repair issues or flows, yet
diff --git a/homeassistant/components/data_grand_lyon/strings.json b/homeassistant/components/data_grand_lyon/strings.json
index 55e36d4c325e..d7d112d66ad1 100644
--- a/homeassistant/components/data_grand_lyon/strings.json
+++ b/homeassistant/components/data_grand_lyon/strings.json
@@ -2,7 +2,8 @@
   "config": {
     "abort": {
       "already_configured": "[%key:common::config_flow::abort::already_configured_service%]",
-      "reauth_successful": "[%key:common::config_flow::abort::reauth_successful%]"
+      "reauth_successful": "[%key:common::config_flow::abort::reauth_successful%]",
+      "reconfigure_successful": "[%key:common::config_flow::abort::reconfigure_successful%]"
     },
     "error": {
       "cannot_connect": "[%key:common::config_flow::error::cannot_connect%]",
@@ -20,6 +21,16 @@
           "username": "[%key:component::data_grand_lyon::config::step::user::data_description::username%]"
         }
       },
+      "reconfigure": {
+        "data": {
+          "password": "[%key:common::config_flow::data::password%]",
+          "username": "[%key:common::config_flow::data::username%]"
+        },
+        "data_description": {
+          "password": "[%key:component::data_grand_lyon::config::step::user::data_description::password%]",
+          "username": "[%key:component::data_grand_lyon::config::step::user::data_description::username%]"
+        }
+      },
       "user": {
         "data": {
           "password": "[%key:common::config_flow::data::password%]",
diff --git a/tests/components/data_grand_lyon/test_config_flow.py b/tests/components/data_grand_lyon/test_config_flow.py
index 966331281b27..ed338880651f 100644
--- a/tests/components/data_grand_lyon/test_config_flow.py
+++ b/tests/components/data_grand_lyon/test_config_flow.py
@@ -193,6 +193,81 @@ async def test_form_already_configured(
     assert result["reason"] == "already_configured"
 
 
+async def test_reconfigure_flow(
+    hass: HomeAssistant,
+    mock_config_entry: MockConfigEntry,
+    mock_get_tcl_passages: AsyncMock,
+) -> None:
+    """Test the reconfigure flow updates credentials and preserves subentries."""
+    mock_config_entry.add_to_hass(hass)
+    original_subentries = dict(mock_config_entry.subentries)
+
+    result = await mock_config_entry.start_reconfigure_flow(hass)
+    assert result["type"] is FlowResultType.FORM
+    assert result["step_id"] == "reconfigure"
+
+    result = await hass.config_entries.flow.async_configure(
+        result["flow_id"],
+        {CONF_PASSWORD: "new-pass"},
+    )
+
+    assert result["type"] is FlowResultType.ABORT
+    assert result["reason"] == "reconfigure_successful"
+    assert mock_config_entry.data == {
+        CONF_USERNAME: "user",
+        CONF_PASSWORD: "new-pass",
+    }
+    assert dict(mock_config_entry.subentries) == original_subentries
+
+
+@pytest.mark.parametrize(
+    ("side_effect", "error"),
+    [
+        (ClientConnectionError(), "cannot_connect"),
+        (ClientResponseError(None, None, status=401), "invalid_auth"),
+        (ClientResponseError(None, None, status=500), "cannot_connect"),
+        (RuntimeError("unexpected"), "unknown"),
+    ],
+    ids=["connection-error", "auth-401", "http-500", "unknown"],
+)
+async def test_reconfigure_flow_errors(
+    hass: HomeAssistant,
+    mock_config_entry: MockConfigEntry,
+    mock_get_tcl_passages: AsyncMock,
+    side_effect: Exception,
+    error: str,
+) -> None:
+    """Test the reconfigure flow shows errors and recovers."""
+    mock_config_entry.add_to_hass(hass)
+
+    result = await mock_config_entry.start_reconfigure_flow(hass)
+
+    mock_get_tcl_passages.side_effect = side_effect
+
+    result = await hass.config_entries.flow.async_configure(
+        result["flow_id"],
+        {CONF_PASSWORD: "new-pass"},
+    )
+
+    assert result["type"] is FlowResultType.FORM
+    assert result["step_id"] == "reconfigure"
+    assert result["errors"] == {"base": error}
+
+    mock_get_tcl_passages.side_effect = None
+
+    result = await hass.config_entries.flow.async_configure(
+        result["flow_id"],
+        {CONF_PASSWORD: "new-pass"},
+    )
+
+    assert result["type"] is FlowResultType.ABORT
+    assert result["reason"] == "reconfigure_successful"
+    assert mock_config_entry.data == {
+        CONF_USERNAME: "user",
+        CONF_PASSWORD: "new-pass",
+    }
+
+
 # Stop subentry tests