Changing the water pump is a basic task, but unfortunately getting to the stage where your are in a position to carry out the operation is not quite so basic
It is advisable to drain the coolant via the radiator and block nipples.....
Stage 1...Remove the auxillary/poly or serpentine belt whichever phrase you use..
http://www.volvoforums.org.uk/showthread.php?t=30475
Stage 2.. Undo your timing belt following the instructions here...
http://www.volvoforums.org.uk/showthread.php?t=30549 up to the end of stage 6....
The belt should now be hanging loose in the bay
Stage 3...Rig up a tool to hold the tensioner pulley and arm tight away from the water pump securing bolts...
The water pump can be seen below the outlet/exhaust cam sprocket.....
Stage 4.... Using a 10mm 1/4" ratchet undo the seven retaining bolts..these are usually very tight as they will have been secured with loctite..ensure the socket is on squarely to avoid rounding off......
Stage 5 When the bolts are out remove the waterpump and using a putty knife or the like ensure that all the gasket is removed from the block...
Stage 6...Compare the old and new water pump...
Stage 7....Apply a small amount of gasket sealant to the block side of the gasket and place the gasket in place over the dowells using the sealant to hold the gasket in place....
Stage 8...Locate the pump to the engine body apply some "loctite blue" and tighten the retaining bolts to 20NM
Stage 9...Replace the belt as shown in...
http://www.volvoforums.org.uk/showthread.php?t=30549