old wash arm was cracked and had gunk in it.

The upper arm assembly removes easily by unsnapping a plastic part or two that holds it in place.
Once the arm assembly is removed, you simply remove the screw that holds the arm on, replace the arm, and replace the screw. The key thing here is that you need a "star drive". This is a screw driver or bit that has a star shaped end. I don't know what size it is.