Step 1:
Free up the thin harness under the harness retainer plate, this plate just pops out of the bottom of the frame.

Step 2:
You have verified it is the harness with the 3 wires blue/red/black.

Step 3:
This is the actual assist sensor you are replacing, behind the left pedal arm on the crankshaft.

Step 4:
Pull the harness down through the bottom of the frame, a little fiddly but not difficult, to give you some slack.

Step 5:
Remove the bottom battery retainer, 2 screws same as the top bracket.

Step 6:
You now have room to juggle the connector through the hole in the bottom of the frame.

Step 7:
Remove the 10mm allen bolt securing the pedal arm.

Step 8:
Screw the pedal arm puller into the pedal arm, turn it in about 5-6 turns. Turn the center puller 15mm nut to pull the arm, remove the arm.

Step 9:
The sensor slips off the shaft with a little persuasion, push the new sensor on the shaft ensuring the splines are engaged and it is flush with the frame, same as it came off.

Step 10:
Reverse the procedure to re-assemble, torque the pedal retaining bolt to about 20 ft pounds.