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 11.3 SRU 10 to Oracle Solaris 11.3 SRU 13 and then realized you need an SRU 12 image, reboot to your SRU 10 BE and update to SRU 12 from there.
Alternatively, you could install a kernel zone or Oracle VM Server at the Oracle Solaris version you need.