Disc brakes dont need to be adjusted and all drum brakes are self-adjusting unless you have an antique..lol...if it is a newer vehicle and you think the rear drum brakes need adjusting then it is a good possibility that the self-adjuster isn't functioning properly. Try making a few hard stops going backwards on pavement as this may help. If it doesn't then take your vehicle to a qualified technician and let them handle it. If it is an older vehicle(referring to something that would be an antique with either 4 or 2 wheel drum brakes) then there should be a slot in the backing plate for you to adjust them. All you would have to do is get an adjuster(found at a local automotive store) place it in the slot and turn the star wheel downwards to tighten(use the opposite if the slot is in the drum and not the backing plate) do this until you feel slight resistance when you turn the drum.