If more than one person uses Garmin Express on one computer, you'll have to associate each device with the correct Garmin Connect account to avoid uploading data to the wrong account:
Steps for a user to access this screen:
1. Open Garmin Express.
2. Select a device from the drop-down on the upper left.
3. Click "Device" on the upper right.
4. Scroll to the bottom of the page and look for "Garmin Connect Account".
If you need to change the account for a device, use the "Change Account" button.