How do I change or replace the buckle on my strap?

The buckle is held in place by a spring bar.

To remove it, stick a small pointed object (like the end of a paperclip, sewing needle, or the pin end of a spring bar tool) into one of the small holes on the side of the buckle. Press the spring bar in and slide the buckle off.

To attach the new buckle, just reverse the process — push the spring bar end in, slide the buckle on, and make sure both ends pop into the holes.