// Get the existing student from the db
var user = (Student)UserManager.FindById(model.Id);

// Update it with the values from the view model
user.Name = model.Name;
user.Surname = model.Surname;
user.UserName = model.UserName;
user.Email = model.Email;
user.PhoneNumber = model.PhoneNumber;
user.Number = model.Number; //custom property
user.PasswordHash = checkUser.PasswordHash;

// Apply the changes if any to the db
UserManager.Update(user);