Before diving deeper on removing 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.
Steps to follow:
To start, navigate to the document's page.
On the document's page, check the name of the document on the list, then click the kebab/three dot menu button.
The sidebar should appear, click "modify expiration".
A calendar prompt should appear with a "remove expiration" button, click remove expiration.
Click confirm and the document expiration should be removed.
NOTE: To verify if the document expiration is successfully removed from the document, check the sidebar if it still displays the document expiration.