Which pump? There are three to four different pumps. If it's a mayflower or Johnson brand pump it's pretty simple. There is a tab lock on the side of the pump case lift that up and twist the pump assembly and it will unscrew. This will be the same with the other pumps in the boat. How size the pump generally the bigger the better.
There are videos on YouTube that can help with this as well.
If it's the water pump on the outboard if you feel comfortable, this pretty easy, but at times a pain in the butt. Look at YouTube for that as well.
I hope this helps.
