No matter how often you try to press F2, F6, F8, F12 or anything: If you have a pre-installed Windows 10 you need to change some settings in the BIOS/UEFI in order to be able to boot from your DVD.
First, you need to get into the BIOS/UEFI menu to change the boot order by pressing Fn+F2. As you directly notice you can't even see the DVD drive.
In the following you should select 'Boot Mode' and chose 'Legacy Support' instead of 'UEFI'.
And in 'Boot Priority' you should also change from 'UEFI First' to 'Legacy First'.
Go to 'Exit Saving Changes' and open the BIOS again. Now you can see a bit more and also the DVD drive like you were used to in the older days.
Finally you should put the DVD in the first position ahead of the HDD. Save the changes and your inserted DVD will boot.
You'll be able now to install Windows 7, 8, 10 or anything from your DVD drive.