git config remote.origin.pushurl git@github.com:aUser/aRepo

git remote set-url --push git@github.com:aUSer/aRepo

$ git remote -v
github  git@github.com:schacon/hw.git (fetch)
github  git@github.com:schacon/hw.git (push)
origin  git://github.com/github/git-reference.git (fetch)
origin  git://github.com/github/git-reference.git (push)

$ git remote set-url --push origin git://github.com/pjhyett/hw.git

$ git remote -v
github  git@github.com:schacon/hw.git (fetch)
github  git@github.com:schacon/hw.git (push)
origin  git://github.com/github/git-reference.git (fetch)
origin  git://github.com/pjhyett/hw.git (push)