Android : Update user fields with Parse

How can I update a user field using Parse as backend? Here is the code:

for(ParseUser user : mFriends) {
    if (user.get("phoneNumber").toString().equals(result.substring(3))) {                                    
        user.increment("field");
        user.saveInBackground();
    }                               
}

The problem is that if I use the current user (the user who's using the application) instead of user, it works, but I want to update the field of another user.