On WIN 10, if you install torch, it becomes your default magnet opener. to change back to previous client, in my case utorrent, deleting torch works, or following
open regedit
HKEY_CURRENT_USER\SOFTWARE\Clients\StartMenuInternet\Torch.BGL7GRC43Z6P6SZKFQCFJGPO2A\Capabilities\URLAssociations "NAME" MAGNET
DELETE ENTRY AND WILL GO BACK TO PREVIOUS DEFAULT IE UTORRENT