I downloaded the map I wanted (UK) and copied it across to the Garmin. The file I used was gmapsupp.img . When I start up the Garmin now it puts up come copyright signs, including for openmap, but I still have the old maps (I’ve checked what I can see on the satnav against the online Openmap.)
I must be missing something - renaming, downloading other files, … ???
What model Garmin are you using? Did it have any maps installed previously?
You might need to go to the “Map Setup” menu, and enable the OSM maps, and disable any other maps.