I'm planning to upgrade TFS from 2013 to 2015. In case something goes wrong during the upgrade installation, I'll rollback and revert every thing to its original state (TFS 2013). I'm performing the upgrade on the same server, which means that if the upgrade is successful, TFS 2015 will override 2013.
Is there an article to follow in case of rollback? Preferably an official article from Microsoft.
You're basically looking at the Restore procedure outlined here:
But before you restore you need to uninstall TFS 2015 and reinstall TFS 2013 and its latest update packs.
If your setup includes SharePoint, Report Server and one or more build servers you may need to revert those too. As far as I know there is no definitive rollback documentation. The recommendation is to do a trial upgrade on a second environment, resolve any issues you may find and then perform the final upgrade.