I have a M17A1 gas mask but the protective lenses are stained dark yellow and the actual lenses are the same I heard that UV rays could've damaged it or something but I wanted to know if there's any way to clean them or if they are permanently damaged 

I don't know that this is UV specific damage. A lot of plastics will yellow with age and it could be from a variety of issues. Towards your issue, I think trying to reverse an aging process is not advisable. You're more likely to cause damage and more than unlikely to have success. 

I served as the NBC NCO for my unit in the early 1980s. If I remember correctly the outer lens for the M17A1 were replaceable. Maybe you can find some NOS replacements out there.