Water pumps are not easy to change. They are especially difficult on a japanese car like the Galant. Unless you have experience, a lot of time, and a good set of metric tools, I would avoid this project. I can't really get into the "what do I have to do" part in changing water pump, but if you really want to change it, you should get yourself a goo service manual. It can give you step by step instruction on performing this job. There are too many steps involved for me to describe it in a little email.

Good luck!