Change default boot disk

1. Select Compute -> Virtual Machines from the left sidebar
2. Select the virtual machine you wish to change the boot disk of
3. From overview page select Disks -> System disks from the menu below the virtual machines hostname.
4. Click the name of the disk you wish to use as the boot disk
5. Select make boot disk? from the Disk details section

The selected disk will now be that virtual machine's boot disk. Reboot the machine to use the new boot disk.