I'm fairly certain that Garmin stores all data in metric. When you look at the data on your device or in Garmin Connect, it is converted as necessary to display it using the setting you have chosen.
What is the actual problem that you are trying to solve?
I have the same issue. The Garmin device is set for kilometres but when downloaded it appears in miles, can't find where to convert it on Garmin Connect
Garmin Connect has is own setting for such. You need to need to click your user name at the top right of the GC page the select settings from the dropdown then you want the tab for 'display preferences'.
