Like many other bicycle brakes, a BMX brake relies on a cable to control the braking action. When the brake lever is squeezed, this cable contracts, pulling each brake arm in toward the wheel. A set of pads attached to the brake arms then grip the wheel's braking surface, causing the bike to slow. Repeated use of the brake causes the cable to stretch over time. Tightening the cable will effectively tighten the brake.
Use a 10mm open-end (or adjustable) wrench and loosen the brake cable fixing bolt. This bolt is located on the brake and "fixes" the cable to the brake.
Loosen the brake cable locknut (where the cable enters the brake lever) and turn the adjusting barrel beside the locknut away from the brake lever. This will allow you to fine-tune cable tension momentarily.
Compress the brake arms (the sides of the brake) so the brake pads are flush against the sides of the rim.
Pull the end of the brake cable taught (while the brake is still compressed) with needle nose pliers, and using the 10mm wrench, re-tighten the anchor bolt over the cable.
Adjust pad clearance (each pad should clear the side of the rim by about three to four mm) by turning the adjusting barrel beside the brake lever in toward the lever. Check your adjustment after each turn. Once the adjustment is complete, tighten the brake cable locknut (beside the adjusting barrel).