Use the point of a spudger to lift the digitizer cable connector up and out of its socket on the motherboard.

To re-attach press connectors like this one, carefully align and press down on one side until it clicks into place, then repeat on the other side. Do not press down on the middle. If the connector is misaligned, the pins can bend, causing permanent damage.

If any part of your screen doesn't respond to touch after your repair, re-seat this connector, making sure it clicks fully into place and that there's no dust or other obstruction in the socket.