I'm not sure if it's possible on all calculator LCDs. There is usually a polarized lense in front of the LCD, which only allows light of a particular orientation through. The LCD segments change the rotation of light that passes through it so that it no longer reflects off the backing, which it looks dark. Some displays have the front lense integrated so that you can't remove it. You'll have little luck inverting those. If the lense is removable, then try rotating 90 degrees in front of the LCD. Ofcourse if it works, that means you'll have to find a piece of polarized plastic to cut into shape.