Change Boot Order option is only available with Kvm, Xen HVM, XCP-HVM, Virtuozzo Kvm, Proxmox Kvm.
You can edit it during creation of vps from iso or you can change it after installation of iso is complete.
You will require to change boot order to avoid rebooting vps again from iso image.
First click on the "edit" option .
You will then get the page to edit all the data of Virtual Server.
There will be a boot option you can just select boot order and then click on "Edit Virtual Server".