Off the top of my head....
Turn the mirror selector to the centre. Put the car into reverse. Turn the mirror selector to the kerb side mirror and then adjust it to a suitable position for viewing the kerb. Turn the mirror selector back to the centre and then back to kerb side. That's it, when you select reverse next time it should dip to your setting - but you have to keep the mirror selector on the kerb side mirror for it to work.
Hope that made sense...