Optane Firmware update require legacy BIOS, XPS8940

Am a happy user of XPS8940, with Intel 280GB OPTANE SSD.

I found that the firmware of the OPTANE is rather old. So, downloaded latest FW package and made ISO disk. Read the user's guide and found that the disk must be booted in legacy BIOS mode. But I could not find the legacy mode setting. How can I do it?