OK, if it's a UEFI Bios thing, follow this from step 1 (ignore the shaded box starting with "i Note:). Doesn't matter if it's not a Dell.
https://www.dell.com/support/article/uk/en/ukbsdt1/sln143038/access-uefi-bio...
Otherwise, it's a generic how to boot from a DVD article for you :-)
Aha! The info re *Windows 7* was useful. So I think we're talkng BIOS here not the UEIF thingy. F2 is my friend, then.
If anyone does fancy writing a side by side BIOS/UEIF comparison though, go ahead. The interweb clearly needs one. imo, ymmv.
Bev.