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I strongly suspect the centre screw physically holds the secondary assembly onto the spider vane assembly, so I wouldn't be undoing it for fear of the secondary assembly dropping onto the primary mirror.
The 3 smaller screws at 120 degrees are the collimation screws that allow the secondary to be collimated. I've never done this on a Newtonian so I'll let someone else chime in with the how to. Also, try doing a Google search on "Newtonian collimation" you should turn up so excellent material.
If you live near a local club, try to get along to a meeting – the fear and mystery of collimation evaporate when you can see it being done.
Cheers
Dennis
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