If this is a VM you could take a snapshot before the SP install. If it goes south recovery is easy just don't delete the snapshot until your sure the SP is good.
If not then you need good backups (I usually stop SQL and make a copy of the system DB's as an extra backout option as copy/paste is easier than restoring the system DB's). If issues then uninstall SQL, re-install to the same location, service pack to the same level before the update, stop sql services and copy the system containers back, start SQL!! Or you can re-install and service pack, start SQL in single user mode and restore system DB's then start sql normally and attach user DB's.