How do I fix U brakes?
There are 2 screws you can see at right side of the brake. the bottom one is just above the pad screw - it's one that holding the cable. Open it a little.
Check if the cable is going smooth (if not, you should oil the cable or even replace the cable, and maybe the housing too). Check the pads, that they are not wear, nor dried on sun.
Then pull the cable (be sure that the cable going well at the lever), push the pads up to the wheel from both sides (to add tension), and tighten the bolt. The upper screw is to add more tension if needed.