From SharePoint Online, click on App Launcher Icon  from the navigation bar

In the app launcher  click on the Yammer tile

Enter your password if required

At the top of the page near the search box, click on the (...) ellipses

From the drop down, select Files

Click the file you wish to preview and edit

If you wish to edit the document, click  Edit File

Make the changes to the document

Close the document when finished editing