Pull requests get stale very fast, and they need to be rebased often.
Please note that they need to be rebased. This is not same as
master to your branch. Please read
to gain a better understanding of the difference between
git merge master.
Rebasing a pull request can be complicated sometimes because of merge conflicts.
If you know how to rebase, then that's great!
If not, here's another way to handle this issue in the least complicated way.