Updates one stored custom field value by its fieldValue id. The request must include the fieldValue's contact and field ids along with the new value: omitting them fails with a misleading 404 that reports "No Result found for Field with id 0" rather than a validation error. Multi-select fields use the ||option 1||option 2|| format. Date and datetime values are best sent in ISO 8601; other formats are also read, as described on value. The response side-loads the updated contact in a contacts array. 403 without contact-edit permission, or, for a user outside the Admin group calling the API directly, when the field sent in fieldValue.field is not on any of the user's lists. Sending a contact and field that already have a different value record fails, because a contact can have only one value per field. Unlike the reads, the preferred language field comes back as its stored code. A value sent for a lookup field, which shows a field of another CRM object and holds no value of its own, is not saved: the stored record is left as it was. It is returned unchanged when it holds that contact's value for that field; otherwise the response carries the contact and field sent with a null value. If your account uses social channels, fields whose service is engage are kept up to date automatically by those channels. A value sent for one is ignored: the stored record is returned unchanged when it holds that contact's value for that field; otherwise the response carries the contact and field sent with a null value.