En primer lugar, hola, voy a escribir en español para que la gente se entere...
It's completely safe as long as you know the exact model and revision of you mainboard. If you have a self-built computer you will have the mainboard instructions book, which will surely tell you the model of the mainboard, then you have to search the manufacturer's web site for the bios file for your mainboard.
Then download it and also download the bios flasher, which a program called awdflash.exe or something similar.
Then you have to create a boot disk containing the bios flasher program and the bios file, start the system using the disk, and flash the bios.
It is completely safe as long as you don't use the wrong bios or you press the power or reset switch while you are flashing.
Even if you screw it up, you can make in another computer a boot disk nearly identical to the one you used, but adding a few command lines, which will allow you to boot the computer and make it work again.
I know this post is very general, but if you've got any doubt...