Just setting up brand new Vista 6400 and I cannot figure out how this adjustment can be made. Screen sits higher on one side of the platen than the other but the nut that looks like you would loosen to correct the problem, looks impossible to get to with a socket wrench or any wrench for that matter. I hope this is a stoopid question with an easy answer

The off contact bolts on the back of the head you have highlighted are used to set off contact and level the print head to the pallet. Use small box wrenches and opened end wrenches to loosen and tighten them.