I have to remove these rivets (see pics).
What tool can I use for that?
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
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.
You may be able to punch out the center using a hammer and a suitably sized punch.
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.
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.
I always drill. Works like a charm, rivet's metal is quite soft. On the third picture, rivet et gone already, you should be able to push it out.