Gently pull the glass panel straight up off the iMac.

The glass panel has several positioning pins around its perimeter. To avoid shearing these pins off the glass panel, be sure to only pull straight up during removal.

Be meticulous about cleaning the LCD and the inside face of the glass panel before reinstallation, as any fingerprints or dust trapped inside will be annoyingly visible when the display is on. Placing the glass flat, inside face down, on a fresh aluminum-foil surface is a good way to keep it clean.