so, I hit the following catch 22 situation:
When trying to update the device software on my edge 840 via Garmin Express I get the error message, that the device has not enough internal memory.
Based on the suggestions I tried to use the map manager and get rid of the not needed maps, e.g. NA and Trails.
HOWEVER, when I try to do this, Express tells me, a software upgrade is needed first, before I can use the map manager and stops the process.
This puts me back to square 1.
According OSX Finder, the edge has 3 MB free space left on the device.