sorry you have to use SaveChanges instead. and use Single() instead of FirstorDefault

public void UpdateMoneyEarned(User user)
        {
            using (TeachableEntities db = new TeachableEntities())
            {
                
                User q = (from u in db.Users
                where u.UserID == user.UserID
                select u).Single();
                q.MoneyEarned = 0
                db.SaveChanges();
            }
        }