XPS 15 9570 Booting an OS from an external USB DVD Drive

Hi Guys. I am currently running the supplied Windows 10 Home 64bit. I want to upgrade to Windows 10 Pro 64 bit using a genuine Windows 8 Pro 64 bit disc and the Media Creation Tool. I want to create a bootable DVD of Windows 10 Pro 64 bit.

How do I change the BIOS settings, specifically to accomplish this? Should I go to Legacy settings rather than keep the UEFI? I don't care about BitLocker too much either.

I want my 1st boot device to be an external USB DVD Drive, followed by the SSD(if that is correct), followed by a USB stick.

After accomplishing that, I want to revert the boot order back to SSD, external USB DVD Drive and finally a USB stick. In other words, I don't want to stuff it up! Thanks for any assistance. If you preferably have a URL to explain this simply, that would be great also.