gmapprom.img

You may be able to delete a file but that won’t prevent the potential of future problems.  Why don’t you add a memory card, or increase the size of what you may be using?  A miroSD card is very inexpensive these days.  See https://www8.garmin.com/manuals/webhelp/edge810/EN-US/GUID-0368F1B6-ABC9-46FE-98E3-91A71012DCA0.html

Didier-Martin said:

There is unfortunately no SD card slot on the edge 840!

Didier-Martin said:

on mu garmin edge 810.

that is confusing. 

Which device is this thread about?

Well you started out saying Edge 810 and now you are referring to an Edge 840.  If indeed you mean the 840 see Not enough memory for map update.  Someone more familiar with the device will have to render some assistance on the file deletion situation.  Or perhaps back up a file to some other device, delete it and then retest. 

It is an Edge 840, apologies for confusion...

Although not applicable to the Edge 840, the content at Removing maps from outdoor device might be of concern.

I used garmin basecamp for this.  Connect Edge 840 on PC and run basecamp. On the left side you see .img name and map name for this .img file. Then find this .img file on the edge 840 and delete it. Or safe on the PC.