There are various ways an upgrade could fail so how you roll back will depend on what systems you have and the point at which it failed.
If an upgrade fails very early on in the upgrade, the installation process will stop and you can restart the system as nothing will have been changed. As it runs further through the upgrade it will start converting tables. If it does stop after this point, the system will be left with mixed Spectrum versions making it unusable. There are various options at this point:
1. If your Spectrum servers are on virtual machines, you can restore the virtual image of the entire server. It is recommended to take snapshots prior to upgrading.
2. If your severs are running on physical machines and you need to roll back to the previous version, then Spectrum needs to be uninstalled and reinstalled. This will require installing the required base image e.g. 10.2.0, 10.3.0 or 10.4.0 and any port patches to bring your system back to your previous release version. It is recommended to note the contents of the $SPECROOT/Install-Tools/.history file prior to the upgrade. It is also recommended to backup the SpectroSERVER, Archive Manager (DDM) and Spectrum Report Manager (SRM) database prior to running the upgrade in case you need to re-install. Also, backup any customization from the $SPECROOT/custom and $SPECROOT/Notifier directories.
It is highly recommended all upgrades are tested in a test environment (mirroring the live environment) prior to upgrading your production environment.