You need to remove the wheel and flip the bike upside down to get good access to the brake. Disconnect the cable. Each brake arm is individually connect to the frame through a center bolt, unscrew that bolt, but be careful, there's a spring under each brake. Once you have this removed, clean all surfaces (brake pin and the brake itself), and relube everything. The spring has two "arms", one that connects to the hole in the brake and one that connects to one of the 3 holes in the frame. You need to move the spring into the 2nd or 3rd position, which will increase the tension that spring is under, and bolt the brakes back on with the center screw. Make sure your brake housing is not too long and pushes the brake to either side, or too short, which pulls the brake to one side.
"Suddenly the thought struck me; my floor is someone elses ceiling" - Nils Ferlin