As of the first day of the quarter, Math 126B is full. No entry codes will be given; the only way to get in is by registering through STAR when space becomes available. Don't give up hope yet: there are always a number of spaces that open up during the first few weeks. Although there are no guarantees, past experience shows that most people who want to take the course eventually get in.
If you want to change sections, you must do it officially through STAR.