Yes, this bothered me when I was trying to install a retail version of Windows 8.1 Pro on my laptop that originally had Windows 8 non-Pro installed. Building on what cl-Albert said, I followed these steps to get my computer to boot from my Windows DVD instead:
1.) Go to BIOS and disable Secure Boot (for me, I found it under "Advanced Boot Options" but yours may differ so just search).
2.) Save and exit changes and your computer will reboot.
3.) Go back to BIOS if it doesn't automatically show up after restarting
4.) Find the option to Enable CSM and then once you turn that on, you would be able to select your boot options like the Hard Drive, Disc Drive and USB if applicable.