When a change to a file in SharePoint Online (SPO) is saved, a new version of that file is created. The following steps will guide you through reviewing and restoring a previous version of a file.

Access the SharePoint Online site's document library through a web browser
Select a document
Select ellipses
Select "Version History"
Touch or click the date/time of the document version you wish to download for review
To restore to a particular document version (revert to that version), select the dropdown arrow that appears when hovering over the modified date/time
Select "Restore"

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .