My PC is trying to use phone's tethered connection causing Internet access to be blocked on my PC. How can I change default PC connection during USB tethering?

You can assign very low priority to the tethered connection. Windows will then only use it when all other connections are not available.

1. Go to Control Panel
2. Network and Internet
3. Network and Sharing Center
4. Change adapter settings (left column)
5. Right click USB connection and select properties
6. Select Internet Protocol Version 4
7. Click Proprerties
8. Click Advanced (don't change other settings and numbers)
9. Deselect Automatic metric and type very high number there (9999, for example)

Screenshot of the settings (click for full resolution):