That’s a rivet. You’d have to drill it out

No. As noted, it's a rivet. It was originally a straight piece of metal rod with a cap (visible in the top image) at one end, inserted into the joint, then the other end deformed with a rivet tool to create a lip on the end (lower image) so it stays in place.

To remove it, use a drill bit about the same diameter as the rivet shaft, and drill it out from the end in the lower image. You usually only have to drill less than a millimeter before the lip breaks free, and you can pull out the rest of the rivet. The trick here is that the rivet is probably hardened steel, that means it'll take a carbide drill bit, and some time.

This is obviously a destructive procedure for the rivet, and then you need special tools to put in another. It might be possible to replace with a screw, but it won't be quite the same.

Rivet. Drill. Enjoy :)

It's a rivet, and it can be drilled out, but replacing it with an equivalent joint is going to be difficult. if you don't use the same material. This looks like a beach chair or some kind of outdoor thing. You might be okay replacing it with something like this: https://www.amazon.com/YORANYO-Assorted-M5-Decorate-Bookbinding/dp/B0B7WMQM97

You may be able to go to a local hardware store and find similar, but in lower quantity and maybe something not as specified for leathercraft/book binding. If you do go that route, put some loctite in the thread so you don't unscrew yourself accidentally and lose one or more.