Brake pads wear in use and need to be replaced before the indicator slots disappear. Rubber hardens with age, so replacing old pads after a couple of years will improve braking performace regardless of wear. Some brakes have removable blocks that sit inside the shoe, these can be replaced independently without the need to replace the entire shoe. Other brakes have an integrated shoe, where the whole unit needs to be replaced.
Remove Old Rim Brake Blocks
Remove any retaining pins or screws that hold the block in the shoe. Slide the block out of the open end of the shoe.
Replace Rim Brake Blocks
Replace the new rim brake blocks into the cartridges. Lubricate them with warm water if they are stiff.
Re-fit screws or pins. Make sure when you remount them on the bike, the closed end points forwards otherwise the blocks may fire out under heavy braking.