If the current default version is broken
Check the list of default versions of the Perf Dashboard. The current version should be at the top of the list, and the previous version should be directly under it.
You should rollback to the previous version. Please add that version to the top of the list after you rollback to it.
Find the target version in the app engine versions list. Select the checkbox next to it, and then hit the “Migrate Traffic” button.
