Rename or recolor an existing tag.
Both fields are replaced, so send the current value for the one you are not changing. Requires the `posts:write` scope.

```graphql
mutation UpdateTag {
  updateTag(input: {
    id: "some_tag_id",
    tag: {
      name: "Summer sales 2026"
      color: "#2AF598"
    }
  }) {
    ... on TagActionSuccess {
      tag {
        id
        name
        color
        colorName
        isLocked
      }
    }
    ... on MutationError {
      message
    }
  }
}
```