Jack up and support the rear of the jeep. With the handbrake off, adjust the toothed wheel accessible through the grommeted hole in the backplate until each brake binds slightly, then back off a couple of clicks. Operate the handbrake lever then recheck the adjustment. If you have no handbrake at all, the parking brake shoes might have disintegrated, so probably best to have a look inside the disc/drum assembly before you adjust unless they've been inspected recently.