If you're replacing existing bib taps with those of a more modern design, it's a relatively simple matter of disconnecting and unscrewing the old ones and fitting the new taps in their place.
However, it's quite possible that you'll want to remove the bib taps altogether and fit a new sink with some pillar taps.
This will involve a little more plumbing work.
To start with, turn off the water supply and remove the taps and old sink.
If the pipework comes up from the floor you'll need to uncover the run in the wall to below where the new sink will go.
You should then be able to ease the pipes away from the wall and cut off the exposed sections.
This will allow you to join short lengths of new pipe, bent slightly if necessary, to link the pipe ends and the tap tails.
Alternatively, if the pipes come down the wall you'll have to extend the run to below the level of the new sink and use elbow fittings to link the pipe to the tap tails.
In either case it's a good idea to fit the taps to the new sink first and to make up the pipe- work runs slightly overlong, so that when the new sink is offered up to the wall you can measure up accurately and avoid the risk of cutting off too much pipe.
Rather than having to make difficult bends you can use lengths of corrugated copper pipe.
One end of the pipe is plain so that it can be fitted to the 15mm supply pipes with either a soldered capillary or compression fitting; the other end has a swivel tap connector.