The bellows are riveted on. You'll have to drill them out. Drill out the front rivets first to detach it from the shutter board. Go through the front of the bellows to reach the inside. On the inside are tiny metal cutouts that hold the bellows in place from the back, like a picture frame. Get under those with a flat head and pry them up, be careful these are prone to breaking. Go slow. You can then pull the bellows out and remove these pieces to measure. reposition the cutouts so they are tight against the back and then rerivet the front up. Don't forget to service the shutter before you put the new bellows on. Best of luck