I ain't no geek, however.....
In a word you can't, as the maps are coded to one GPS device only. You can have the maps on as many computers as you like (I have mine on three) but only on one device at a time. This was a change brought in a few years ago to stop exactly what you are doing.... One bod buying the detailed and copying it onto all his mates' devices.
But.... If you Google Garmin maps for free... You may find a UTube film telling you how to get, well.... Garmin maps for free. It's theft of course but hey, it's only software.
Some bods use something called 'Open maps' or 'Open streets' or summat like that, which I think are free and certainly detailed enough. You could download those to an sd card and go from there, I guess?
The whole thing is slowly but surely becoming irrelevant as most new devices are being sold pre-installed with Lifetime maps anyway.