If you reached this page from a warning during an upgrade from a stable version then it's probable that some of our branch changes have affected you.
If you intend to stay with the most recent stable version of Moodle, then you need to switch your checkout over to use the appropriate MOODLE_3x_STABLE branch (such as MOODLE_32_STABLE, MOODLE_33_STABLE, etc).
If you are using Git, then you must create new local branch that will track the remote stable branch and switch to it.
git checkout -b MOODLE_39_STABLE origin/MOODLE_39_STABLE git pull
Replace 39 with the relevant Moodle version. At the first line, the new local branch MOODLE_39_STABLE is created and set to track the remote branch of the same name. At the second line, the git-pull command fetches recent changes from the upstream and merges them into your local branch. To update your Moodle next week, you can run just this second line.
For more about how Moodle versions are managed in Git, see our dev:Moodle_versions page.