title: Update branch without deleting pull request made from it
body: <p>I am new to git and GitHub. I forked a repository and cloned it to my machine. Created a branch <code>fix-a-certain-issue</code>. I worked on this branch and pushed it back to <code>origin/fix-a-certain-issue</code> and then opened a PR. Two days later, the PR is reviewed and I am supposed to make changes to it. The problem is my <code>fix-a-certain-issue</code> is behind <code>upstream/master</code> by <code>x</code> commits. How do I update <code>fix-a-certain-issue</code> so that it is even with <code>upstream/master</code> without deleting/closing my PR?</p>