After an event is finished, the timeline is automatically transferred to the archived tab for future reference. It's important to note that these archived timelines cannot be edited directly.
In case an event is postponed or requires editing after it has already passed and been moved to your Archive due to the timeline item dates being in the past, a solution is to change the event's date to a future one. This will "unarchive" it and allow for further editing.
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 under the Active timeline tab 🎉
It is important to note that if you open an archived timeline, and then try to change the date within the timeline, the system will not allow you to change the date or any other items within the timeline.