About suggestions to update a pull request branch
If you enable the setting to always suggest updating pull request branches in your repository, people with write permissions will always have the ability, on the pull request page, to update a pull request's head branch when it's not up to date with the base branch. When not enabled, the ability to update is only available when the base branch requires branches to be up to date before merging and the branch is not up to date. For more information, see "Keeping your pull request in sync with the base branch."
Managing suggestions to update a pull request branch
On GitHub.com, navigate to the main page of the repository.
Under your repository name, click Settings. If you cannot see the "Settings" tab, select the dropdown menu, then click Settings.
Under "Pull Requests", select or unselect Always suggest updating pull request branches.