There is a nut in between the caliper and the frame that the spring passes through. It should be able to be rotated with a 13mm (or so) cone wrench, or offset brake wrench. It looks like you would want to turn it counterclockwise to move the caliper from right to left, hold that nut in place and torque down the mounting nut behind the brake arch. Make sure you have a starred washer between the caliper and the frame to keep it from rotating.