Most of the time, the "yellowing" is because of deposits that get into the film. It's not actually the plastic, but road grime.
First, clean it with this:
http://www.xpel.com/products/Tools_p...ducts&pn=R1331
Then, seal it with this:
http://www.xpel.com/products/Tools_p...ducts&pn=R1326
If the cleaner doesn't take the yellow out, chances are that it's permanently stained and you'll have to get it replaced. The clear bra is not a maintenance-free solution, but with the right care, it'll last quite a while.