You can usually access the belt on many belt-drive turntables through a couple of access holes in the platter, just under the rubber mat. The platter usually pulls right off, and for reistallation of the belt, you have to reach through these holes and sort of thread it back on.

But some models are different than others, and it sure can't hurt to have the owners manual. If you don't have one already, I'd probably spend a few bucks with one of those guys above and get the manual.