Hello what you need to do is reconfigure the boot priority to boot from the external dvd drive.
The first step should be getting into the BIOS. When you turn on the laptop you need to press a key like del or F10 to enter the BIOS.
Then you need to find the boot order and ensure that USB CD/DVD drive is at the top. Instructions for navigation and selection should be on the screen.
Ultimatley what you need is the BIOS configured to boot from the external drive.
- Let us know if you need any help.
If you are uncertain about anything you have done in BIOS exit without saving changes otherwise save changes and exit.