Renames opportunity extra fields. Send only the fields you want to rename:

{
  "extra_fields": {
    "com.marketcircle.daylite/extra1": { "display_name": "Membership" }
  }
}

Fields you leave out keep their names. Send {"extra_fields": null} to reset every field to its default name. Renaming a field only changes its name; values are not affected. A successful request returns 204 No Content.