To downgrade your operating system release, boot into a BE older than the version you want to downgrade to, and upgrade from there. For example, if you updated from Oracle Solaris 11 11/11 SRU 10 to Oracle Solaris 11 11/11 SRU 13 and then realized you need an SRU 12 image, reboot to your SRU 10 BE and update to SRU 12 from there.