Upgrade?
If you are using Tumbleweed, this is rather a downgrade… 
You should be able to change your root password without a DVD though.
Press ‘e’ at the boot menu, and append “init=/bin/sh” to the line starting with “linux” or “linuxefi”, then press ‘F10’ to boot.
You should get into a minimal text mode system, where you should be able to just run “passwd” to change root’s password to a new one.
Any clue of how do I make the DVD to boot first ?
Did you actually burn a bootable ISO to your DVD?
Can you read the DVD otherwise?
Your BIOS might just fall back to the hard disk because it cannot read the DVD or it is not bootable.