I am following the below steps to change the boot option in my Dell Inspiron 7520
Switch on the machine and hit F12 to get to BIOS.
Disable secure boot in the boot options screen.
Enable the load legacy option ROM.
Keep the boot list option set to UEFI.
Press F10 to save and exit.
Shut down the machine and start it again with the USB device attached.
Press F12 on boot to get the legacy boot options.
Pick USB from the list, and off you go.
