Assuming you've been developing a feature on the feature branch, but master has changed since you started work.
This is depicted below:
- - - master
|
| - - feature
To make a Pull Request, you will first need to merge master into feature.
First, ensure that the local master is up to date. Then, checkout the feature branch.
If in doubt, git merge master then git push will work.
If you don't want to have merge commits, you can rebase using git rebase master and push with git push --force-with-lease.
Make sure you don't git pull between the rebase and the push because it can cause changes to be merged incorrectly.