Hi Gokhan,
In order to resolve your issue, what you can do is? Save doc Id of the user in local pref. at the time of creating profile. Once that document id saves in local prefs you can further use it for updating the document. Please have a look at this link which will help you to update that JSON document based on docId and let me know if it helps..
I will be Happy to address your concern.
Regards,
Himanshu Sharma