You can view the version history of a journey by clicking on the Version # of the current journey at the top left of the Journey canvas.
Select a previous version to view the old journey in a view-only state. (Changes cannot be made to previous versions.)
To "Restore" a previous version, click on Restore and confirm. Once done, that is now the live version of your journey.
Restore options is not available on Scheduled Journey triggers.
Remember: Once a contact enters a journey, they are attached to the version of the journey they entered. So when you restore a previous version of a journey, only new entrances will start using that version.