function update(req, res, next) {
  const user = req.user;
  user.username = req.body.username;
  user.mobileNumber = req.body.mobileNumber;

  user.save()
    .then(savedUser => res.json(savedUser))
    .catch(e => next(e));
}
Update existing user
@property {string} req.body.username - The username of user.
@property {string} req.body.mobileNumber - The mobileNumber of user.
@returns {User}