Hello there mike.
Changing the water pump involves loosening the timing belt tensioner so the timing belt will need to be replaced as well. I always recomend the tensioner be replaced as well because it's just cheap insurance.
Do you need full repair instructions?
There is nowhere in the internet that I know of to get BMW repair instructions.
These are the instructions for the timing belt. Once you get the belt off, just unbolt the water pump. Make sure you pay attention to the timing and get it set up correctly.
I REALLY DIDN'T WANT TO GET IN TO MESSING WITH THE TIMING BELT/TIMING
IF IT'S POSSIBLE TO CHANGE OUT ONLY THE WATER PUMP.