Updates a custom field definition by its field id. Only the properties present in the request are changed, plus udate and updatedby on every call. perstag is normalised on save (uppercased, A-Z, 0-9 and only) and regenerated from the title when blank. Changing type does not convert stored values or remove existing options; switching to textarea sets rows to 5 and cols to 40 when they are 0. Options cannot be managed here: options and relations keys in the payload are silently ignored. Manage options with the fieldOptions endpoints and list relationships with the fieldRels endpoints. 404 if no field has this id; 422 if title is emptied, contains HTML or is already used by another field, or if perstag is already used by another field or by a system personalization tag. A field cannot be changed into or out of type lookup; other type changes are unaffected. field.lookup replaces a lookup field's configuration, and is refused on a field of another type. Only for an account the crm-lookup-fields feature is on for; otherwise any request naming the lookup type or carrying field.lookup fails with 400.