Begin by locating the ground or negative cable on the battery (yellow arrow) and use a 13mm wrench to disconnect it. Place the cable in a manner that it cannot accidently touch the battery post while you are working.
The starter is located on the lower left side of the motor (red arrow). It attaches to the bell housing. Removing the starter motor from its mount will make a difficult job so much easier. You do not need to disconnect the wiring to the starter just get it out of the way. Give the starter a quick wipe down if it is oily as it is heavy. You do not want to drop it once it is free.
There are two 17mm bolts holding the starter motor to the bell housing (red arrows). It is a very tight fit but using a 17mm socket you will be able to remove both bolts. The motor will be free with the bolts removed so do not let it drop.
With the starter pushed forward out of the way it is very easy to remove the sensor. You can just see the top of the sensor (red arrow). Use a 5mm Allen and remove the single nut holding the sensor to the engine (yellow arrow).
Pull the sensor (red arrow) out from its mount and disconnect the sensor from the wiring connection. Note: on some older sensors you need to disconnect the sensor from the wiring where it plugs in in the engine bay.
This photo illustrates the sensor out from the engine (red arrow). There is a small magnet on the end of the sensor. There should be nothing attached to this magnet. Installation is the reverse of removal.