I want to change the Git default remote branch destination so I could just
git remote rm origin
fatal: No configured push destination. Either specify the URL from the
command-line or configure a remote repository using...
git remote set-url --push myfork origin
git config push.default upstream (or matching)
Another technique I just found for solving this (even if I deleted origin first, what appears to be a mistake) is manipulating git config directly:
git config remote.origin.url url-to-my-other-remote