[Tom from Drippin']

On the two I've done, which involved repainting so I wasn't concerned about harming the finish, I slightly spread the channel and used a small screwdriver as a chisel with a small hammer to dislodge the old rubber. It came out in both large and small chunks. I seem to remember that it's also held in place by the channel being staked in several spots. You may find that you need to open the channel slightly to install the new rubber and restake to secure.