how do i replace a tapaa?

Turn off the supply for that tap. Keep the tap on to drain any excess water from that part of the system.
The fixings for the tap are beneath the basin/sink.
You might need a basin spanner/tap wrench to reach the nuts as their may be limited access for a regular wrench. About a fiver at B&Q.
There should be 2 separate fixings. The top nut closest to the basin attaches the tap to the sink. The bottom nut attaches the supply pipe to the tap. There may be an O ring in the bottom nut. Now is a good time to replace it.
Put down cloths underneath the sink to mop any water in the pipe.
Undo the bottom nut first, then the top nut. After you've unscrewed both nuts the tap should detach easily by pulling upwards.
When installing the new tap make sure you use any silcon spacers supplied. One should go between the tap and the basin surface, the other one underneath the basin before any fixings.
These are to help prevent any damage to the basin surfaces.
Don't over tighten the top nut as you will crack the basin!!
After you replaced the O ring and tightened the bottom nut you can turn the supply back on.
Only turn it on half way then check the bottom nut for any leakage.
Tighten it up if it leaks.
If there is any weeping you may have to undo the bottom nut and run 3 turns of PTFE tape around the thread.

Big Softy
April 2007