//Switch to the branch from which you created the pull request:

git checkout pull-request-branch

// Overwrite the modified file(s) with the file in another branch, let's consider it's master:

git checkout origin/master -- file_path_from_project_root_folder

// Commit and push it to the remote:

git commit -m "Removed a modified file from pull request"
git push origin pull-request-branch