Mongoose: Find, modify, save

The user parameter of your callback is an array with find. Use findOne instead of find when querying for a single instance.

User.findOne({username: oldUsername}, function (err, user) {
    user.username = newUser.username;
    user.password = newUser.password;
    user.rights = newUser.rights;

    user.save(function (err) {
        if(err) {
            console.error('ERROR!');
        }
    });
});