Replacing a wall-mounted mixer is perhaps a simpler operation than replacing a mixer mounted on a washbasin.

Let's see in detail:

1. First of all, as always, close the central faucet
2. Remove the old mixer by first unscrewing the right nut then the left one.
3. Brush the attachments on the wall then give a few rounds of Teflon tape on the fillet
4. Place or check that there are gaskets inside the fixing nuts
5. Replace the new one by alternately tightening the fixing nuts