You will most likely need to tighten the tension on the brake cable to increase the braking power. Over time the shifter and brake cables will slowly stretch out, causing reduced performance from the brakes and derailleur. Tuning/re-tensioning your bicycle can help keep these areas performing better.
To adjust the rear brake, you will need a size 8 metric socket or wrench to loosen the nut that is shown in the attached image. Push the black "arm" forward just a little bit and pull back on the cable, while keeping these two parts tensioned - re-tighten the bolt.
The rear brake may also require additional lubrication to help with the band brake mechanism that makes the system work. If so, you are able to use a high temperature/high pressure grease, normally found at any bike/auto shop.