HardwareSecrets published a guide on how you can restore yellowed plastic parts to their original color, you can learn it over here. It involves using a homemade peroxide-based solution called Retr0bright and UV light bulbs.
"Plastic parts of older computers become yellow or brown over time, so you end up with a computer that looks yellow or brown instead of white or gray. In this tutorial, we will show you how to restore old plastic parts to their original color by using a homemade peroxide-based solution called Retr0bright."
