Call their number (800-360-7561) and tell them the problem (i.e. the pointer frequently freezes, or the mouse no longer works, or the LED no longer turns on, etc)...if you sound like you're a newbie, it's probably better than sounding as though you heard that you can get a free mouse replacement if you call (doesn't sound like you're doing it just to get a free mouse). The call lasts less than 5 minutes, most of which is spent giving them your shipping info.
I've heard it takes less than 3 days to get the replacement, but I just put in for one a few days ago and they said they're backordered for about 2 weeks