If you’re seeing the following error when you run the command

$ git remote add origin git@github.com:username/Hello-World.git
fatal: remote origin already exists.

or

fatal: The remote end hung up unexpectedly

then these steps will solve your issue

$ git remote rm origin
$ git remote add origin git@github.com:username/Hello-World.git
$ git push -u origin master

First step removes the origin remote which has an incorrect reference.
The next 2 steps will add and push the changes again.