I've tried both methods.
Update via USB stick sequence, not all points documented:
Start PC
Put in USB stick
Browse to Volvo website
Download map file
Save to USB stick
Remove USB stick
Put USB stick into car socket
Install
Update via car's SIM, sequence, pretty much everything is listed
Ensure box is checked in Auto-update of default home map. Need to do this once on car purchase.
Do nothing again, map will update when required.