“how to update it’s json files to be compatible with frappe 12 ?”
Yes a json file defines a DocType’s data Model schema at a point in time. So with each schema change, the json advances too from one version definition to the next. But no need to change the json yourself as normally a patch directs each schema change and any required data migration in step.
In the case of a custom app, with custom fields added to standard DocTypes and custom DocTypes that you introduce, patches for these that involve schema changes, you must implement yourself.
These threads give pointers to how a patch is developed
To ‘push’ or ‘port’ custom fields defined in your custom app to their standard DocType you need to investigate fixtures and hooks for example