Using the larger of the included hexagonal keys, undo the three bolts on top of the extruder. TIP: Leave the bolts in, rather than taking them out. Then they are harder to lose. NOTE: There may be a small amount of foam adhesive joining the flat, white ribbon cable to the extruder. You may remove this during the process.
To remove the levelling arm, undo the three screws holding it onto the extruder. Then you can slowly lift the sensor up and out of the extruder. NOTE: The levelling sensor is on the end of an arm, so be mindful of it hitting things.
Once you've removed the levelling sensor arm, you will need to disconnect the two connectors attached to it. These are a white signal connector, and the black servo connector.
Next, remove the single screw located in the middle of the levelling sensor. NOTE: There is a plastic washer on the screw, please don't lose it.
Position the new levelling sensor, and use that same screw, and plastic washer to secure it back to the plastic arm.
Reconnect the two connectors back into the extruder.
Slowly and gently slide the levelling arm back through the extruder, being careful not to catch any wires as you go. Feel free to push any stray wiring around the best spot as you do this. Then use the three screws from earlier, to secure the levelling arm back onto the extruder.
Slide the black cover back onto the extruder, making sure the white FCC cable is fitting correctly. Then simply do up the three bolts of top, and you're finished.
Complete both an 'Extruder calibration' and 'Level', as the printer will need to adjust to the new sensor.