Download the bios to a blank FAT32 formatted USB stick, restart your machine, spam DELETE to get into bios. Find the bios flash option, point it to your USB stick, and let the bios update function run. DO NOT turn off or otherwise touch your PC while it is updating as you can brick your machine this way. Once it is finished, you will have the latest bios.
Please note that if you have made changes to your bios of any kind, they will be erased and everything will return to default in the new bios. If you had any tweaks to CPU or RAM etc, you will need to do them again.
You should never update your bios from inside Windows. Windows is an additional possible layer of failure during the bios flash, which is a very critical operation, as a crash or power loss during bios flash will result in a bricked bios which means a bricked motherboard. On many ASUS boards the bios chip is socketed meaning a replacement can be ordered from ASUS to de-brick the board but that's just not a situation you want yourself in, in the first place.
Merry Christmas, or if you don't do Christmas, happy holidays.