Apply a heated iOpener to the screen to loosen the adhesive underneath. Apply the iOpener for at least 5 minutes.

The display adhesive of the Galaxy S21 Ultra is extremely strong, and it's likely you need to reheat and reapply the iOpener several times before starting the display removal and during the removal procedure.

A hair dryer, heat gun, or hot plate may also be used, but be careful not to overheat the device.