If all you want to do is remove the old boards and replace with new ones, then vehelpfulguy's link is fine, though I would add that it may be useful to insert a piece of 1/8 " plywood between the crowbar and the wall to minimise any damage to the plaster. I agree with him, that it's not always as straightfoward as it might seem, because until you've removed the boards, you don't know what you're letting yourself in for.
Normally, one would use cut (floorboard) nails to fix the new boards and drive the heads just below the surface of the boards, using a punch. The small depressions can then be filled and sanded to provide a nice flat surface to paint over.
If you feel that nailing the boards is not an option, then there are plenty of suitable adhesives, usually applied with a sealant-gun, that would do the trick. This is only good if the plaster is sound. Check with B & Q.
Good luck with it.