Fisrt you should just try using either F9 or Esc while booting and already have your usb plugged in before you power up. Those two keys are often used by HP for there boot menu options.
Second you could try going into the BIOS setup menus and seeing if you can enable the option from there. F2 or F10 will get you there most times.
If your gonna try a BIOS update first find out what revsion you have(i see v.5.08 is posted), then check hp's website for a newer one, review every bios update that came after yours to see if such a feature was placed in as new or fixed. If not then flashing the bios would be pointless for the most part.
If you still want to anyways, just download the file, look through for insructions or readme files before you attempt for specific instructions. Some will allow you to make a bootable cd/usb but alot of them now run through windows.
HIGHLY RECCOMEND that you be plugged into a known good battery backup before you try a bios update.