Removing the display requires cutting through the adhesive around the perimeter of the screen. After the adhesive is cut, it cannot be used to re-seal the display in place, so you'll need to apply a new set of adhesive strips.
This guide includes steps showing how to cut a notch in the adhesive strip, allowing the iMac’s microphone to function properly. If your adhesive strip already contains a notch, [https://www.ifixit.com/Guide/iMac+Intel+21.5-Inch+Display+Adhesive+Strips+Replacement/137411|follow this guide instead].
Some images in this guide use an older iMac, which has minor visual differences. These differences do not affect the repair procedure.