The APIM is different between NAV and non-NAV SYNC 3 units. So it’s not just a software update.
According to the Ford 2016 brochure, Limited’s came with SYNC 3 without NAV, it was an optional or part of the 301A package.
You can find NAV APIMs for sale, but they’ll also require reprogramming to your as-built data. There are people that do it, I just don’t recall specifically where off the top of my head.
The big question is whether factory NAV is worth the cost versus just using Android Auto or CarPlay for navigation purposes.