2002 Ford Focus How do I move the belt tensioner to get a n

Based on the way you wrote your post, seems like you are able to twist the tensioner to relieve some belt tension, but not enough to get the belt off the pulleys.

Get an assistant, go thru the motion of pushing the tensioner to relieve pressure off belt, and then twist the crankshaft one way or another while pulling the belt off a convenient pulley. Careful not to snag your fingers.

I've encountered this too, and in my efforts to get enough slack, I actually broke off the aluminum 18(?) mm casting on the tensioner itself. (A bit too much pressure, eh?). Tensioners differ by design based on Focus application, but twisting crankshaft pulley works for me.

Getting new belt on (if real tight again): Either reverse the procedure, or try the next size longer belt. If you choose the longer belt, be sure tensioner is actually creating tension on the belt. Otherwise, you'll end up with a loose and noisy belt.