how to uninstall maps from forerunner 945

if i recall correctly, this has been addressed by garmin in the fenix 6 forum; long story short, don't uninstall them, you have plenty of space on the device ;-). if you don't need them -> disable should do the trick, 

I recently bought a FR 945 in North America, but then moved to Europe. I want to get the Topo Active Europe on the watch, but it doesn't have enough space and I am told not to uninstall the Topo America that came with the watch. Any advice for this?

Uninstall the Topo America.

Is there a tutorial on how to do that, and the Garmin Express warns me not to uninstall pre-installed maps

Delete the files on the watch that are like over 1gb if you want more space for music or whatever 

You can un/install the from Garmin Express. The pre-installed maps refer to the Course maps. Otherwise this was posted some while back - https://forums.garmin.com/sports-fitness/running-multisport/f/forerunner-945/228013/can-i-remove-maps-from-watch

Delete the biggest files in the Garmin\Maps folder, these are all the map files. If you delete them, there's not much more than 32 GB or so on the watch left.

Then you can install the latest maps via Garmin Express with your Garmin connected to the USB port of your computer. Note that Garmin Express is tricky and sometimes fails for no reason, so you may need more than one attempt. It did not work on my laptop but after several attempts it then worked on my main computer. Make sure everything is on the latest version before installing, watch and Garmin Express.