Those often slide into a channel from one side of door and slide accross to other side. If you are able to remove door then turn bottom side up and soak with soapy water help's.

My husband and I replaced that on our shower door a number of years ago. I remember we took the door off. We had installed it ourselves and had the instructions in a file folder of home items. I no longer have the instructions because after he passed away I followed the directions in reverse and removed the door. I replaced it with a rod and shower curtain because I hated that door. It always seemed to leak

Success! I was able to get it started with a metal dental hook tool and once I’d got about an inch of it out of the channel the rest just pulled right up. I cut the end off at the edge of the frame with a utility knife.

Where did you find a replacement seal?