Lift the bottom edge of the motherboard up and pull towards the bottom of the phone to remove it.

You may feel a bit of resistance. This is normal, since the midframe is slightly bonded to the heat sink with thermal paste.

During reassembly:

Move all cables that connect to the motherboard (battery, display, interconnect, and antennas) out of the way so they don't get stuck under the motherboard when it's reinserted.

Reinsert the motherboard at a downward angle to position the headphone jack and proximity sensor back into their recesses.