If you look at Love 500 on YouTube, a video will be uploaded at 9am today (Wed 28th April 2021) which shows how to remove the rear side panel. In that video I show how to get that bolt out on the bottom of the seatbelt. Be careful, watch the video and you'll see why. It has a habit of snapping.
You must take that bolt out to release that bottom bolt to get the seatbelt out.
However, if you are reupholstering the seats you don't need to take it out. There is also a video on there of how to remove the rear seats.