In the admin review cycle card view, you can click the 3 dots for more options around the selected review to find the option to extend it.
In the list view, the extend icon will be on the right hand side of reviews that are able to be extended, along with the other icons.
If you need some more time for your teams to finish their reviews, you can extend the end date of the cycle as long as a few key elements are true:
If your cycle is a draft, is already closed out, or hasn't started yet, you won't be able to extend it.
If the cycle doesn't have calibration as part of its manager step you don't need to extend it. If the manager step closes in a cycle with no calibration, you can continue to edit and submit your manager reviews until an admin closes the cycle manually.
If you are extending an Open Review Cycle with all the steps running at once, extending the cycle will change the end dates for all steps in the cycle.