Edge 840 storage for maps; deleting maps

Hi I have a year-old Edge 840 which is updated to the latest version yesterday. My device is not recognised by Garmin Express, so I need to update maps by WiFi. The top of the map manager screen says "-128MB", and I am wondering if this prevents me from downloading map updates which don't seem to occur. 

Other Forum questions have talked about deleting maps for other regions to free space - I can not find anywhere to do this.