It isn't too difficult. First go to the Asus website for your specific motherboard. Then under the Support menu select the BIOS from the Download menu. Then follow the instructions. Download the BIOS to the root directory of a USB flash drive.
Then enter the BIOS of your system. insert the USB drive. Go to the Menu item for updating the BIOS. Browse for the BIOS on the flash drive, select it and press enter. That is it.
The whole process is detailed in the manual for the motherboard.