追加
$ git remote add new https://github.com/xxxx/xxx.git
登録確認
$ git remote -v
new https://github.com/xxxx/xxx.git (fetch)
new https://github.com/xxxx/xxx.git (push)
名前変更
$ git remote rename new upstream
$ git remote -v
upstream https://github.com/xxxx/xxx.git (fetch)
upstream https://github.com/xxxx/xxx.git (push)