I just did this not long ago. I ended up putting a very thin flat head screwdriver under the outside ring and prying it up just slightly, then push it back down leaving the center pin head exposed. Pull that center pin out with pliers (or more often just the screw driver and a well-placed finger) and it pops right out.
On mine, the outside of the rivets that went through the wood behind the shower panel have a texture to them that gives them some grip, so even after I removed the center pin, I still had to pull them pretty hard to get them out.
They all went back in just fine though. Just take your time and be careful you try not to break them or bend them. They're not expensive to replace or anything, but I would imagine finding a match would be pretty difficult on short notice.
2014 Jay Flight 28 BHBE
2015 RAM 2500 6.4L HEMI, Tradesman 4x4, 3.73
Blue Ox SwayPro (BXW 1503)
Upgraded from an REI internal frame backpack and a Eureka 1/2 dome tent!