Isolate the relevant water supply.
For hot and cold water valves remove the cover from underneath the handle. There is no cover for the middle handles.
A small grub screw should be visible remove this using an Allen Key, the handle should now slide off the valve.
Unscrew the valve with either a 17mm or a 15mm spanner/socket depending on the valve. Insert and tighten the replacement valve.
Refit the handle making sure it is vertical when in the off position, if the lever is not 100% vertical the valve may need a little more tightening to ensure the lever lines up. Tighten the grub screw, then refit the cap if applicable.
