Like other parts on a bicycle, brake pads suffer a certain degree of wear and tear. Friction caused by braking reduces the thickness of pads, lessening their ability to grip the sides of the wheel rim. Most pads are outfitted with grooves that help monitor this wear. When these grooves begin to disappear, it's time to remove the pads and fit new ones in their place.
Release tension on the brake. Many road brakes today feature a quick-release, a small lever or button found either on the brake or beside the brake lever that allows you to "open" the brake so that there is ample clearance between the brake pads and the wheel.
Use a 2mm Allen wrench to loosen the small Allen screw that passes from the brake shoe through the brake pad. The shoe is the outer, metal surface of the pad assembly.
Slide the pad from the brake shoe. You may need to grip the tip of the pad with a pair of needle nose pliers in order to free it.
Fit the new pad in place. There will be a small groove in the backside of the pad through which the 2mm Allen screw passes. Be sure this portion of the pad is aligned with the Allen screw hole in the brake shoe.
Replace the 2mm Allen screw, and tighten in place. Repeat the above steps for the second brake pad (each set of brake contains two).
Close the brake quick-release, restoring tension to the brake cable.
Release tension on the brake by squeezing both sides of the brake and lifting the cable from the body of the brake.
Use a 5mm Allen wrench and loosen the brake pad fixing bolt. This bolt secures the pad to the arm of the brake.
Remove the brake pad from the arm of the brake. Be careful, for this manner of brake pad may contain a pair of washers (small metal disks) that can be easily lost. You'll be reusing these washers when fitting your new pad.
Fit the new pad in place on the brake arm. If the pad included any washers, be sure those are first in place (the new assembly should look exactly like the old one) prior to installation.
Fix the pad in place by tightening the fixing bolt with your 5mm Allen wrench. Repeat the above steps for the pad on the other side of the brake.
Squeeze both ends of the brake and place the cable back into its anchor inside the body of the brake. This will restore tension to the cable.