Question: how to upload tcx to strava?
Answer: Login to Garmin Connect and select the "Activities" link to give you a list of all your uploads.
Select the activity. On the activity page, click on the link that says "Export" and choose the "TCX" option. ... 
Upload the . tcx file to the Strava website. ... 
To upload files in bulk, select up to 25 files at a time.