curl --request PATCH \
  --url https://api.example.com/api/v2/contact/{contact_id}/attribute/{api_name}/{item_id} \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "is_primary": true
}
'

{
  "data": {
    "id": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
    "envelope": {
      "id": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
      "email": "jsmith@example.com",
      "is_primary": true,
      "labels": []
    }
  }
}