Export files from Garmin Connect in .tcx, .gpx, and .fit format
On a computer, log into your Garmin Connect account.
Use the left-hand toolbar to navigate to your "Activities" page.
Click on the activity (not the check box) that you need to export to be brought to the activity's detail page. (note: if you only see the option to export the csv file you may still be on the activities list page. You will need to click on the specific activity. If you see the activity details include the map, you will have the full list of options below.)
Use the gear icon in the upper-right corner to view your export options.
Choose the format you wish to export. If your device records natively in .FIT format, and you wish to export that version, use the "Export Original" option.
The files you've exported from Garmin Connect can be uploaded to Strava here.