Follow the steps to open the required file in viewer.

You can navigate to view superseded revisions of the file open in file viewer from the dropdown option at the top containing details for each available revision, as shown in below screenshot.

Any file revision, if marked as private and not shared with you, won't be accessible to you unless you are an administrator on the folder containing that particular file.

Click on any of the previous revisions to open that particular file revision in a new browser tab.

Whenever the file revision opened in viewer is found as a superseded revision, a message will appear at top side of the screen stating 'You are not viewing the latest revision of this file'. This indicates that you are viewing an older revision of the file.