How do i clean my iphone 7 silicone case if it has yellow stains?

This is the recommended way as published by Apple. Let us know how you get on.

https://support.apple.com/en-gb/HT204172

Write to Apple feedback.

http://www.apple.com/feedback/iphone.html

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you just can't clean it and here is my advice for you

Don't buy the light color cases because it will get stained you just get the dark it's better

btw apple is offering cases and it's very good i bought one 2 years ago and still having the same shape and color.

Cheers