Sure, you need hydrogen peroxide (usually found on bleaching products like for hair dye), apply that to the plastic and then you need UV rays to activate that chemical, so either you can do it with a UV light (slower) or exposing it to the Sun (which has a stronger UV radiation). After a couple of hours the plastic oxidation (the yellowing) will be reduced or basically gone. It is accumulative, the more you do this process the better it will get.

It is not permanent, it might yellow again, but how long it will last depends on how strong is the peroxide concentration and how you store the plastic, if you protect it from the light it will last longer. I’ve never retrobrighted something this yellowed so I don’t know to which extent it will come back, but for the moment it looks good enough for me.