Whenever a team is about to expire, you'll receive a notification within the Chat tab of Teams Manager. At what point along the Lifecycle progression this notification arrives is entirely dependent on your Lifecycle setup.
When you receive this chat notification, you can click on Extend team to lengthen the team's lifespan as needed.
Alternatively, you can extend the lifecycle through the Admin Dashboard.
Here's how that's done:
Navigate to the Teams tab within Teams Manager.
Switch the toggle in the top right corner to Admin view.
Find the desired team under the Active tab.
Click the … button to the right of its name.
Select "Extend lifecycle" from the menu.
Even if you missed the extension window, you can Recover archived teams from the Chat tab within Teams Manager, or the Archived tab within the Admin Dashboard.