Here's just my second general how-to video for LEGO maintenance. I use hydrogen peroxide (H2O2) solution to safely & almost completely reverse yellowing of old parts. White & gray parts show their age in the most obvious way, but this technique works on all colors, and I'm especially happy with past results with blue pieces and transparent windows & windscreens.
Your mileage may vary! In this video I show exactly what I, personally, do, and I also show the very real results. It's important to be attentive during the process, as there are a number of easy mistakes that can negatively affect the outcome. I hope this is helpful!