The brake levers are adjustable.
 has/should have been tightened to keep the adjustment mechanism from re-adjusting by itself
1. Loosen nut nr 1 (if possible) to loosen adjustement screw
2. Turn adjustment screw nr 2 so it comes out of the brake lever to tighten the cable so that the wheel spins freely when brake lever is not pressed
3. Tighten nut nr 1 to secure the adjustment screw
4. Verify brake function in safe environment before use in traffic
If this isn't enough, the brake cable needs adjustment/refitting at the brake pad end. If you are unsure about how, I'd recommend visiting the local bikeshop to get their help or ask for information as incorrect installation of the cable might cause the brakes to stop working completely.