When you install or upgrade Symantec Protection Engine, LiveUpdate is enabled by default to run every two hours. You can modify this schedule, or you can run LiveUpdate manually.
When Symantec Protection Engine performs a LiveUpdate, the definitions that are downloaded are automatically selected as the active definitions. However, you can revert to the previous versions of the antivirus or URL definitions. The definition set that you choose remains active until the next LiveUpdate or Rapid Release update occurs, which then becomes the active definition set.