It is an incredibly bad idea to be picky about which section you want of a high-demand high-enrollment lab/lecture course like Physics 2a,2b,50,51,52. Be glad you were lucky enough to get a seat at all.
If you are already enrolled and wish to change sections of lecture or lab, please follow the instructions for obtaining an add code (Permission number). Those switching sections have no special priority over anyone else who is trying to add.
We can't just switch your section or save your seat for you. By changing sections, you run the very real risk of losing your seat entirely. Do not hope for someone to drop, because a lot of add codes are in play, and these sections are only going to fill up. If you're switching into a lab that has a lot of seats available, then you should be OK, but you still have to follow the usual adding procedure.