Updates a custom field's label, hasFixedOptions, allowedValues and/or displayAs. At least one field must be provided. type is immutable and cannot be changed. hasFixedOptions and allowedValues must stay consistent (a fixed-options field needs non-empty allowedValues; a free-input field must have none), and displayAs: AGE is only valid on a DATE field. A field belonging to another organization, or one that has been deleted, returns 404.