I am trying to change a door knob but first I need to remove the lock. There seems to be a kind of nail that is visible on both sides of this piece (see pictures), but it is stuck. I tried to hit it gently with a hammer and another nail to push it from the side where the head is smaller but it does not work, it is still stuck.
How can I remove this and which tools do I need?
Added 2 more pictures. I have no other pictures but the rest seems standard to me. On the side of the door there is a metal plate with the pieces that go in and out the frame (sorry about the lack of right terms, even in my language I don't know how to name these). On the exterior the knob and system are different: the knob does not have a nail and there is a key lock instead of this round metal piece.
I would like to remove the "golden" plate and the knob but keep the same lock. Question is can I remove this round piece and how without disassembling the whole system inside the door.
Set the pictures to original size (bigger) and added another picture.