Welcome.
As far as I know, in 2015 the infotainment system on the CLS changed from NTG4.5 to NTG5*1. So your car might have either of the two, though chances are that it has the newer NTG5*1.
What functionality are you missing? The maps data is regularly updated, the current version is 2020 v14.0.
There are also firmware updates for your COMAND unit, and you may be able to activate Apple CarPlay / Android Auto.