How do I contact the users of my application?

Only if you have explicitly built in a means for them to supply that information...

EG: If you have a purchase process that includes collecting their email (along with consent to store that and use that) then you can contact them that way.

There is not, nor is there ever likely to be (on the grounds of GDPR and other restrictions, not to mention the technical challenges of detecting a user who not only downloaded but hasn't subsequently uninstalled) any way for you to contact through Garmin. 

Only if they have contacted you with Contact Developer.  The other way is add a note in either the description of "what's New"

And if your app is in anyway collecting that info and you have access to it, be sure to note it in the app's "Privacy Policy"

You could also have the app do the communication.

That would be actual real users. (But they'd have to download the latest version or the app would need the notification feature built in.)

Thanks guys !