Before diving deeper on extending an existing document expiration, please see to it to follow the following pre-requisites:
The document should already have a set expiration.
The user should be also the author of the document in order to modify the expiration.
To start, navigate to the document's page.
On the document's page, check the name of the document on the list, then click more button beside it.
The sidebar should appear, click "modify expiration".
A calendar prompt should appear and when it does, select the new date for the document's expiration.
Click modify expiration to confirm.
NOTE: After successfully extending the document's expiration, you could verify the expiration date by clicking the more button. Check the document expiration section on the sidebar if the displays the exact date, you previously selected. Additionally, already expired document could be also extended by following the same procedures.