C875 How to recover from a failed upgrade?

1.  Make sure the device whether can turn on after update fail. 
 
2.  If the device can turn on, please try to this FW 1.21 to downgrade check if can recover the device?
https://drive.google.com/open?id=13Wgr0LbxwhVxsfSqz8UC9YdN16CaVRIs
 
3. If it still not solves your problem, We are sorry to hear that within the warranty period if you have an item defect issue, you need to contact the original retail store and ask for repair or replacement.
 
4.  Here is Our Global Dealer of the list for your reference.https://www.avermedia.com/us/support/stockist