There is a rod going from the pedal to the back of the tractor. On this rod, you'll find a coupling nut (the longer one) and a jam nut. Loosen the jam nut and then adjust the coupling nut to increase or decrease the length of the rod and therefore adjust the brakes. Get the jam nut tight once you're done with it in the end. You'll want some free play on the brake pedal, like an inch or so.
If you have troubles finding the linkage, get someone on the tractor just pressing and releasing the brake pedal while you check for the linkage underneath.