You will need to disassemble your rails to replace the inboard rail seal. The seal slides into a channel that runs the full length of the rail. Once it is in, we pinch the walls of this channel at both ends to hold the seal into position.
You will need to find those pinch points and use a tool, such as a screwdriver or some other prying device to release the seal.
You can slide the new seal in being careful not to stretch the seal. You can rub soap on the seal to help it slide in. Pinch it at both ends so the seal does not come out. Below is a photo showing the tool we use to pinch the rail.