Those are rivets (specifically pop-rivets), they are a permanent fastener not intended to be easily or routinely disassembled and reassembled.
There are many ways to remove them
Drill
You can drill them out with a HSS drill bit of appropriate diameter for the stem of the rivet. Once you have drilled out the centre, the heads will fall out.
Punch
You may be able to punch out the center using a hammer and a suitably sized punch.
Grind
You can grind off the head using an angle grinder or something similar. Once the head is removed, the remainder will fall out or can easily be pushed out.
Chisel
Some people say they chop off the heads using a chisel and hammer. I guess they mean a cold-chisel of the type intended for use on metal, not the type of chisel intended to be used on wood.
Damaged screws are extracted by drilling into the screw with a drill bit, then using a special screw extractor bit that is tapered and has threads which turn opposite to those of the screw.
The extraction process should be done slowly and carefully because the extraction process is more fragile than the normal insertion of a screw.
I've found that using a hot air paint stripper works.
Use the concentrating nozzle and a low to medium setting. Then when the paint is soft use a flat bladed screwdriver to first take the paint off the surface and then from the slot itself.
Once the head is free from paint a further heating of the screw will also help as there will be a differential expansion of the screw and hinge (or whatever the piece is) which helps loosen the screw.
A further trick I don't always remember is that it can help to try to tighten the screw first before removing it. This might just be enough to break the rust/paint/whatever is causing the screw to stick.
Best Answer
Those are rivets (specifically pop-rivets), they are a permanent fastener not intended to be easily or routinely disassembled and reassembled.
There are many ways to remove them
Drill
You can drill them out with a HSS drill bit of appropriate diameter for the stem of the rivet. Once you have drilled out the centre, the heads will fall out.
Punch
You may be able to punch out the center using a hammer and a suitably sized punch.
Grind
You can grind off the head using an angle grinder or something similar. Once the head is removed, the remainder will fall out or can easily be pushed out.
Chisel
Some people say they chop off the heads using a chisel and hammer. I guess they mean a cold-chisel of the type intended for use on metal, not the type of chisel intended to be used on wood.