Unfortunately you’ve hit a bit of a roadblock due to how timecode is handled in Canon footage coming from older versions of FCPX to now.
Once the project is in that state, the best option would be to reimport and replace the clip in the edit.
The other option- go back to zero timecode- would be to open the file with QuickTime Player 7 (which means you both have to download that separately, and activate it with the Quicktime Pro key). https://support.apple.com/kb/ht3678
Then delete the timecode track on the file (and save the file, of course). If it’s one clip used multiple places—that might be a more practical option, but it also involves more steps.
In new projects created in 10.1 and on with Canon footage this shouldn’t be an issue.