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Despite being documented here , the UserStore#update method does not exist in Kinvey 3 for Android. This new method replaces the old (Kinvey 2) User#update method , which is no longer included in version 3.Thus, I'm currently unable to update a user after login/signup.
Furthermore, the migration guide  does not even mention the changed update method.
P.S.: I already opened a ticket (#3866). The first answer showed that Kinvey support obviously didn't even read my ticket (default answer describing how to create a Kinvey app), so I'm posting this to the forums in case anyone else found a solution or experiences the same issue. If Kinvey support decides to finally really read my ticket and provides an answer, I'll post it here.