First, you need to buy a tube of Rubbing Polishing Compound, made by the TAMIYA company. Its called TAMIYA POLISHING COMPOUND, and should be easy to find online. If that wont work, I have had luck with ultra fine polishing compound that I purchased at a Kragen Auto Supply store called MEGUIIAR’S SCRATCH X.
You will also need a nice, SOFT polishing rag. Do not use paper towels, they are too abrasive. Put a little polish on the cloth and rub it on the bubble. Rub gently, and remember that its better to do it softly a few times, instead of hard once. Apparently, this also helps make bubbles look less yellow. Even though it technically wont remove the yellow, the clean bubble will make it less noticeable.
For vehicle parts, the TAMIYA POLISHING COMPOUND mentioned above is specifically made for clear plastics in model kits. That would be the one I would use.