how do i replace pvc pipes?

UK fittings can be a combination of both screw fitting and glued.
If the existing PVC pipes are part of the waste trap, (Thats the S or P-shaped white PVC that is attached directly to the basin plughole) then they should be screw fitting. You can replace them quite easily with new pipes of the same size. They should be supplied with silicon rubber O rings to form a seal.
You should be able to undo & fasten the screw fittings by hand. If not use a pipe wrench.
Make sure you have a bucket underneath the trap before you start. And don't empty the bucket into the sink when the trap is off. An easy mistake to make!!
But why replace?
If it's due to leakage at a joint, then new O rings should solve that. And it never hurts to give the external thread 2 or 3 wraps of PTFE tape just to make sure, though it's usually not needed.
If it's due to blockage, cleaning the disassembled parts is easy. Use a good detergent, hot water, and an old toothbrush. (I always keep my old toothbrushes in my toolbox, but then I'm strange like that!)
While you've got it off you might as well replace the O rings.
Keep the bucket under the replaced trap and run the tap for 2 or 3 minutes to check the seal.
If what you're referring to is coloured grey and after the trap, then it's the waste pipe.
Cut this pipe with a hacksaw. Use light/medium sandpaper to key all ends to be glued.
Measure, cut, and assemble glueless to confirm it all fits together. This is bonded together with PVC cement, and once it's glued, it's glued for good!
Apply to both sides of the join and hold together for 30 seconds and that should be it.
The joint between the trap and the waste is a simple screw compression joint. Simply push the grey into the white and tighten the collar.

Big Softy
June 2006