FirebaseUser user = FirebaseAuth.getInstance().getCurrentUser();
String newPassword = "SOME-SECURE-PASSWORD";

user.updatePassword(newPassword)
    .addOnCompleteListener(new OnCompleteListener<Void>() {
        @Override
        public void onComplete(@NonNull Task<Void> task) {
            if (task.isSuccessful()) {
                Log.d(TAG, "User password updated.");
            } else {
                Log.d(TAG, task.getException().getMessage());
        }
    });