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You can accomplish this a couple different ways. One would be to go into the BIOS menu and permanently change the first option to CD/DVD, depending on the type of drive listed by the computer. The other would be to change the change boot device for that particular instance. For either option, you’ll press an [F] key to change what the computer boots into immediately after the computer powers on. Each one is probably different.
Generally speaking, the [F2] key usually takes you into the BIOS menu where you can change the boot order in the boot section of the option menu. Otherwise, the [ESC], [F10] or [F12] keys could all possibly take you to the option menu where you can change the order for that particular boot. Try to pay attention to the splash screen the PC first powers on. Otherwise, just experiment!