$ git fetch origin
*From github.com:username/repo*
* [new ref] refs/pull/1000/head -> origin/pr/1000
* [new ref] refs/pull/999/head -> origin/pr/999
$ git checkout pr/999
Branch pr/999 set up to track remote branch pr/999 from origin.
Switched to a new branch 'pr/999'
$ git commit
$ git push -u origin branch
$ git checkout -b newbranch from => 'pr/999'
$ git commit --allow-empty -m "your commit message"
$ git push -u origin newbranch
In this way you can modify someone else pr and send pr to master branch again with the changes.