This image shows the display connector underneath the logic board with the board removed. The next two steps show how to disconnect this connector from the logic board.

Make sure the logic board rests in place against its screw posts. The posts will properly align the board to the display connector underneath it.

Continue with reassembly until you've reconnected the battery. Try to power on the iPhone to see if the screen's connected. If the screen fails to turn on, follow the screen guide to manually reconnect the display connector.