`$ 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`


##### To check out a particular pull request:
`$ git checkout pr/999`

`Branch pr/999 set up to track remote branch pr/999 from origin.`

`Switched to a new branch 'pr/999'`

##### Now modify the files from the pull request, then
`$ git commit `

`$ git push -u origin branch `

##### If the above does not work then,

`$ 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.**