Use a heat gun or an iOpener as a heating tool. This guide explains how to use the iOpener.

Evenly apply heat to the edges of the touchscreen to loosen the adhesive. Continue to Step 3 once the edge of the screen is warm to the touch.

Do not let the iOpener/heat gun remain on one spot for too long. Excess heat can damage the phone.

When reassembling the phone, use the iOpener/heat gun to help the adhesive fuse to the new screen. Heat the screen edges, apply pressure to the screen, and hold the screen in place until it is secure.