Hello,
Have you check in the field information on your application group if the field has the option "Updateable" or not?
If not, then you cannot update your field... if yes, then the error is elsewhere!
Is the field you are triying to change is an "Application ID" field? If yes, then you cannot change the value of this field too, well to change it would be to export/reimport in a new application.
Cheers,
Alessandro