If you have plastic that is yellowed, it is important to clean it as soon as possible. This will help to remove any built-up dirt and debris that may have collected on the plastic over time. To clean the plastic, first start by using a mild soap. Be sure to use enough soap to cover the entire surface of the plastic. Then use a dry cloth to clean the plastic in areas where dirt and debris has been build-up. Finally, use a vacuum cleaner to clean any areas that are difficult to reach with a cloth or soap.