you go to your manufacturer web site download it, , look at instructions and follow them, most of the time you jump to bios and its is in the util section have the new bios on your c:\bios to find it easily with the menu.
many will state it is dangerous, I perform firmware and bios upgrades on computers, printers and various other devices I service in my daily work.
I only have had 1 single motherboard not recover after it was updated and it turned out that board was having "issues" before, and replacing the bios chip repair the system so it wasn't the bios upgrade process but the hardware that was at issue. Can you bricked your system if you incorrectly perform a firmware or Bios upgrade, of course. there is always a risk, and depending your motherboard, many (but not all) now a days have a dual bios to have a back up just for that purpose, again I refer you to your user manual of your manufacturer motherboard.
easiest way to read your bios version is get Speccy, load it and click on motherboard it will list the number right day.