The OS update is in fact firmware update (or software update), this needs to be done via STAR so it's a dealer-only job (or specialist with STAR). The firmware itself is free, but some dealers will charge between half-hour and one-hour for labour (some don't). On later models the firmware update for the infotainment system is done by MB remotely 'over the air'.
There were very few cases where the new maps required a firmware update as a prerequisite, but as a rule the maps can be updated any time and do not require the firmware to be updated first.
With regards the free maps update, please ask the dealer to provide you with the PIN code for the latest version asap. After the 3 years laps, the dealer will no longer be able to generate free PIN codes on MB VeDOK.
The maps can be downloaded from MB, but it's a ~50gb dowoad and dealers prefer to order it from MB on USB media rather than download it themselves.
If you prefer, you can download it at home then copy it to USB media and install it yourself (using the PIN code provided by tbe dealer).
Also, make sure they generate the PIN for the
latest version - the system on your 2018 W205 (pre-facelift) is COMAND Online NTG5*2:
NTG5*2 Maps V15.0 released on 08.06.2021
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