If formula based on cell value

Not sure if an "IF" formula is the right one needed here, but this is what I am trying to do. In cell B9, I want text to display based on what is in cell I6. I have I6 set up as a list of one of the following AS, EA and PI (acronyms for Assemblies, Each and Per Install) 

I am trying to get the worksheet to do the following

• If I6 is blank, I want B9 to read "Select Unit"
• if I6 is AS, I want B9 to read "Assemblies"
• if I6 is EA, I want B9 to read "EACH"
• if I6 is PI, I want B9 to read "Installs"

This is what I started with, but I cannot figure out how to include the rest of the conditions - 

=IF(I6="AS","Assemblies","Select Unit")

Any help would be appreciated. 

Thanks,