How to unscrew a screw that I can't access frontally?
Use an angled (or offset*) screw driver:
Image courtesy of chelseaclockwallpaper.blogspot.com. First reasonably small image I found for what I was after
There are also ratcheting right-angle screwdrivers such as:
Image courtesy of harborfreight.com
If you don't have enough room for even that, use pliers to grip the head. You might use locking pliers (ViceGrip™ is one brand name) depending on how tight the screw is in.
*As noted in a comment by Greg Hill. Thanks!