Hey mate, I'm hoping I can help you here as I think mine are Platoons... (got them for free from a facebook comp so.. xD)
Basically you just gently but firmly pull the outside rim from the lens, If you find a way to do this without getting fingerprints everywhere let me know...
There is small notches in the lens and small pieces that the notches lock onto all around the rim. If you look at your spare lens you will get an idea of where the notches are located. You just pull the rim away until you hear a click and then slowly move around the lens, when you get the notches you will know. It is kind of fidly to do but the goggles will flex quite a bit so don't be scared to get in there.