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Short answer: you can't. Files (map data, firmware updates etc.) are always downloaded to %ProgramData%\Garmin\CoreService\Download (typically C:\ProgramData\Garmin\CoreService\Download)

The only solution I could find is to move that folder to another partition (make sure to completely exit Garmin Express first), then create a link to it:

c:>cd C:\ProgramData\Garmin\CoreService

c:\ProgramData\Garmin\CoreService>mklink /J Downloads "d:\Garmin.Downloads"
Junction created for Downloads <<===>> d:\Garmin.Downloads

c:\ProgramData\Garmin\CoreService>

Since Apple moved from JPEG to HEIF as default format about two years ago, there has been a lot of progress.

There is a very straightforward way to add minimal HEIC/HEIF support to Ubuntu:

sudo add-apt-repository ppa:strukturag/libheif
sudo apt-get install heif-gdk-pixbuf heif-gimp-plugin heif-thumbnailer

That's it! You don't even need to logout or restart, the new functionality is available immediately.

More details here and here.