suspended ceiling
 

I am remodeling my rec room and the existing suspended ceiling isn't square. How can I correct this problem? Can I correct it while it is in place or do I need to take it down and start again.

What do you mean by square? Is it the perimeter tiles that are not square?

If any cuts you had to make on the main runners of the frame are all equal at the cut-end...it has to go up square. It's automatic. This doesn't necessarily mean the tiles on the edges will be square.:)

It depends on how, "off "it is. All the main runners are suspended on wires. You can only shift them so far without actually moving them.
You can try removing the 2' perimeter rails and all the tiles. Shift the main runners so it sets parallel to the walls and reinstall the ones that fit and cut new ones that do not.
You will need new tiles.
They will not match in color unless you replace them all or spray paint them.

The end cuts on the mains will only "go up square automatically" if your walls are perfectly square. Not usually the case. Measure an equal border on either end of the room from the wall to the center of the main runner. Make a small mark on the wall angle.Take a piece of thin string (available at HD or Lowes in the ceiling dept.) and run the length of the room. You can tie the string to a small nail (I usually use a pop rivet) and push it behind the wall angle. This will get the main straight. I can assure you the tees going to the wall will vary in length. Do the same thing with the border tees. Pull a string that direction also. Your main ends will vary also I can almost guarantee. You can trim the ends somewhat to try to square things up, but you're limited by the width of the wall angle (7/8"). It is best to pop rivet all of the main ends and every other 2' or 4' tee to the wall angle to keep them from moving so things stay square. You may get by with adjusting if the ceiling isn't too far out....
Best of luck.