vyos@Vyos-Granite241:~$ set system image default-boot
Possible completions:
  <Enter>       Execute the current command
  1.1.8         Default boot image name
  1.2.0-rc5-


vyos@Vyos-Granite241:~$ set system image default-boot 1.2.0-rc5-
Default boot image has been set to "1.2.0-rc5-".
You need to reboot the system to start the new default image.

vyos@Vyos-Granite241:~$ reboot
Proceed with reboot? (Yes/No) [No] yes

The system is going down for reboot NOW!241 (pts/0) (Mon Oct 29 12:52:19 2018