You cannot "undo" a campaign that has run.
The question is what are you trying to undo? Emails cannot be undone, but you can undo data changes and some other things.
1. Create a new campaign
2. smart list is "Member of Smart Campaign IN "X""
3. Flow - whatever it is you need to undo/redo. Use Choices because you'll probably need to fix that.
You could also just look at the Member list and just select those leads and do a direct flow action. That's only good for a quick data change.