When you make a change to a live (i.e. published) Journey, the "Revert to last publish" option will appear in this menu at the bottom of the canvas:
This option allows you to restore your Journey's configuration back to what is currently live. In order words, it will undo changes since the last time the journey was published.
To see what is live, click "Live View" in the top right hand of the screen:
Note that Journey undo doesn't cover everything. See below for more details.
What does Journey undo cover?
To explain this, let's work through an example. Assume that you have the following Journey live:
(a) Shapes added or removed
If you deleted the Show Headsup Message action, then undo would put it back.
If you added a new shape, then undo would remove it.
(b) Outcomes added, changed or removed
If you changed the email outcome to "On Open" then undo would restore it to "On Click".
Equally, any new outcomes would be removed by undo, and any deleted outcomes would be restored.
(c) Shape configuration
If I selected a different email ("Nurture") on the Send Email action, then undo would restore it to "Lead Follow Up".
If I turned on the option to make the email transactional, then undo would turn it back off.
What doesn't Journey undo cover?
Any change that is made to an asset that has its own publish process is not covered by Journey undo. One such example is email (notice the publish button below):
This is because such assets exist outside of a particular Journey, which allows them to be used in multiple Journeys. In the case of email, please use the dedicated undo button in the email editor instead: