This is for an Zumo XT so hopefully the Zumo 396 may work in the same way. This is on a Windows machine
click on Zumo XT
Click on Manage maps
At your own risk!! Untick the UK and Ireland maps you have and the middle east maps and then hit apply changes. This should remove all of the maps from your unit so that you have space to apply European maps. and leave only the base map on the unit. and in the case of my unit POI's and TopoActive Powersports.
If you hit apply changes the light blue chunk of data disappears off the unit which in my case is the detailed CN Europe maps

And it frees up a big chunk of space.......but you have no detailed maps only the base map and in my case POI's and the TopoActive powersports.
If you want to risk it confirm the Garmin warnings that you are disappearing your maps.
You will get confirmation that you have gotten rid of your maps
Unplug your unit from the USB port to your computer and plug it in again to make changes. (it has to reboot.) and confirm your maps are unticked and the free space shown above is available.
When you plug in your unit again and go to Manage maps under My Maps City Navigator UK and Ireland should still be in the menu but unticked. In this case because I had CN Europe previously installed this is what appears in my menu. Ignore your unclicked Middle East and North Africa Click on Change region under UK and Ireland.
If you have removed your Middle East and N Africa maps you should have space on your unit to select "City Navigator Europe" as instead of "City navigator UK & Ireland" then hit confirm
It should start downloading all of Europe.
When its finished downloading reboot the unit and the maps should be on there. You can check by going to Settings>map and vehicle > scroll down to myMaps and you should have CN Europe NTU 2026.30 ALL North and CN Europe NTU 2026.30 ALL South on the unit and ticked.
Good Luck.