To edit a reel, navigate to the Reels page. At the top right of each reel is a menu (three dots). Simply click that, and choose "Edit."
Once you do that, you will be taken to the Builder page where you'll see the reel you just chose to edit. You can proceed to make any changes you want to your reel, or click the "Discard Changes" in the bottom left if you change your mind about editing your reel.