Before you begin, discharge your iPhone battery below 25%. A charged lithium-ion battery can catch fire and/or explode if accidentally punctured.
Power off your iPhone before beginning disassembly.
Your phone's rear cover may have two #000 Phillips screws or Apple's 5-Point "Pentalobe" screws. Check which screws you have, and ensure you also have the correct screwdriver in order to remove them.
Remove the two 3.6 mm Pentalobe or Phillips #000 screws next to the dock connector.
Use the edge of a plastic opening tool to pry the cellular antenna cable up from its socket on the logic board.
De-route the cellular antenna cable out from under the metal fingers attached to the logic board.
When reconnecting the antenna wire be VERY certain that the round connector at the end of the wire is centered on its mating half on the motherboard before applying pressure. Failure to do so will likely destroy both halves of the connector.
Use the edge of a plastic opening tool to disconnect the Wi-Fi antenna cable connector from the logic board.
When reconnecting the wifi ribbon connector to the mother board be VERY certain that the round connector at the end of the ribbon cable is centered on its mating half on the motherboard before applying pressure. Failure to do so will likely destroy both halves of the connection.
Carefully lift the logic board from the end closest to the speaker enclosure and slide it away from the top edge of the iPhone.
Remove the logic board.
Before reassembly, be sure to clean all metal-to-metal contact points on the logic board (not the mating halves of connectors) with a de-greaser such as windex or isopropyl alcohol. The oils on your fingers have the potential to cause grounding issues.
There's a small rectangular rubber pad at the top of the logic board that prevents the display cables from chafing against the edge of the board. If it comes loose during your repair, make sure to position it correctly when reinstalling your logic board.