Clean yellow phone case?

Hi! Have bought a softcase phone case for my Huawei, today after 3 months it is already totally yellow / brown discolored…

Can you clean it somehow? (Shell has an emotional value)

Something comes when you smoke. Mine is still like new after almost 5 months.

That was not the question

The normal, comes from the UV light.

Can't be cleaned.

This not only comes from smoking, but can also come from it, when the phone is charging or generally hot.