Let's say one of your events got postponed, and the timeline went to your Archive (because the timeline item dates are now in the past).
There is a super-easy way to reactivate your timeline — i.e. to change it back from Archived to Active. Here's how:
Go to your Timeline Archive, and find the affected timeline.
Click the reactivate icon.
Enter a new event date.
Click "Reactivate Timeline."
Now you'll find your reactivated timeline in your Active timeline list :)