Fixing range finder on a Minolta Hi-Matic F

Hi,

I bought a couple of small old rangefinders. One of them, a Hi-Matic F looks great, but can not focus on infinity. So I guess it should be opened and adjusted. My problem at this stage is to get off the top plate. The winder needs to be unscrewed somehow.

I would be grateful for any tips on how to get past this hurdle without buying any special tools.

Thanks in ahead
Erik