You can force the update and to change the train by entering
Code:
freenas-update -v -T FreeNAS-11.1-PRERELEASE update
in the shell.
After updates are applied, you will need reboot to start up 11.1 beta1 on next boot. From there you should also be able to set the train to follow as FreeNAS-11.1-PRERELEASE in the future.