Format HD and install windows 8.1 from windows 8

I have Windows 8 in my desktop PC and want to format my hard disk's partition where windows 8 is and install Windows 8.1. For this, I have a bootable USB with Windows 8.1. When I try to access BIOS to change the boot order of the PC, pressing DEL (as I always did without problem in previous versions of windows) sends me to a blank screen. The BIOS never load. I tried waiting more than 30...