To get the details of replacing just the rubber pad in a cartridge brake shoe we have created two posts.
The first one leaves the cartridge brake shoe on the V-brake arm and just replaces the brake pad. This is quicker and works on replacing the pads that have not been worn down that much. You can find the details here: 'How to replace your V-brake rubber pads? (Option 1)'
The second option removes the cartridge from the brake arm, as it is easier to remove and insert the brake pad that way. It might take a little bit longer but might make for a more precise setup. You can find the details here: 'How to replace your V-brake rubber pads? (Option 2)'