If you are using a dell computer/laptop, please refer to this tutorial to set your computer to boot from CD/DVD-ROM. There are also some differences in various BIOS model and DELL series products, but the key steps are the same. If you cannot find the example to match your computer, you can take one of the examples for reference. Now you can see show you how to change the boot device priority.
1. Press F2 (How to Enter Your BIOS SETUP via BIOS Key) when you reboot your DELL PC.
2. BIOS setup screen appears. Select "Boot Sequence" option, Use ↑/↓key to highlight CD-ROM Device and press Space key to enable it. Press +/- key to move it to the first boot device. When you successfully complete the setup, hit ESC to exit and reboot.
After rebooting, the PC will boot from CD/DVD disk.