Conditional Formatting in Label Expressions

I need help with the syntax with my label expression. I have a specific set of parameters I need to display but I need to change only one attribute to label as something else.

I have AC pipeline that I need to label as NM. How do I get everything else to label but have anything that equals AC change to NM?

A quick work around for me was to duplicate the layer and do a Definition Query and do a simple label expression. My other layer excluded my AC mains and labeled them normally. I know there is a way to build a more complex syntax to complete this task without having to duplicate layers and create Definition queries. Here is the VBScript I have currently.

I am assuming is it would be an if then else statement...not sure how to actually write it correctly though.