Replacing a wall-mounted mixer tap is perhaps an easier operation than replacing a mixer tap mounted on a washbasin.

Let's look at it in detail:

1. First of all, as always, shut off the main water valve
2. Remove the old mixer tap by unscrewing the right nut two turns first, then the left one.
3. Brush the wall fittings clean, then wrap a few turns of Teflon tape around the threads
4. Place or check that there are gaskets inside the fixing nuts
5. Install the new one by alternately tightening the fixing nuts