The Motherboard manufacturer website releases the BIOS updates and gives a description as to what the update does.
In the past it was normally done using a Bootable DOS diskette but nowadays mobo manufacturers come with a Windows method.
The basic concept is that u have a Loader Executable and a Bin file. The Bin file is the actual program that will be uploaded to the BIOS chip. The Loader puts it there.
It's takes less than a minute to update the BIOS BUT I suggest having the device u updating plugged into a UPS cause if power goes during the update - that's the end (unless u have some self-recoverable BIOS like what Gigabyte and i think MSI has).
Step by Step instructions should be on your Mobo manufacturer website