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To update a custom field for a contact, use the Update Contact (PUT /api/contacts/{id}) endpoint. Provide the customFields array containing the fieldId and the new value. If a custom field is not included in the payload, its current value will be retained. Example Payload: 
{
  "customFields": [
    {
      "fieldId": "your_custom_field_id",
      "value": "New Updated Value"
    }
  ]
}