Update Nav system

2013 Yukon XL Denali Some time ago I got mail about updated DVD for my car. New DVD was $150.00. I could get it from them or dealership. I've decided to update the maps and get the DVD. Ask my dealer for it and they looked at me like I was crazy! The tech said there are no updates for your navigation system (he looked in some book).

Question one, how do tell what version (DVD) I currently have?
And
Question two, where can I buy it?