Whitening yellowed plastic

[Livingoffroyalities]

I loaned a portable air conditioning unit to a relative which was left under a window in a sunny spot and now the top of the unit has discoloured yellow.

Is there anyway I can get rid of the discolouration and turn it back to its original white state?

The material is plastic and I am guessing I should not remove the cover.

TIA

You need to cover it to remove the yellow.
You may have some luck with bleach lessening the colour change
Plastic (and oil based paint) turning yellow is a chemical reaction.