Updating motherboard firmware is occasionally necessary and carries real risk. It is worth doing deliberately and worth not doing without a reason.
Do not interrupt a firmware update
A BIOS update interrupted by a power cut, a reset, or switching off can leave the board unable to start. Recovery may require a replacement firmware chip or a manufacturer return. Do not update during a thunderstorm or when power is unreliable, and do not touch the machine while it runs.
When to update
To support a processor newer than the board. This is the most common legitimate reason, and it is the trap: applying the update may require a processor the board already supports.
To fix a specific documented problem you are actually experiencing.
To apply a security fix the manufacturer has published.
Not simply because a newer version exists. Firmware updates occasionally introduce new problems, and a working machine has little to gain.
BIOS flashback
Many boards offer a feature, usually called BIOS flashback or Q-Flash Plus, that updates firmware with no processor or memory installed. The board needs only power, a correctly named file on a USB drive, and a specific port.
This is worth checking for at the point of buying a board, particularly if you are pairing a newly released processor with a board that has been on sale for a while. Without it, a board too old for your processor cannot be updated without borrowing an older one.
Doing it
Download the file for your exact board model from the manufacturer site. Board models differ in ways the name does not always make obvious, and the wrong file will not apply, or will apply and cause a problem.
Read the manufacturer instructions for that board. The process differs between manufacturers and occasionally between models.
Note your current BIOS settings first, particularly the memory profile and any fan curves. An update generally resets everything to defaults.
Use the built-in updater, reached from the BIOS itself, rather than any utility that runs inside Windows. Updating from within a running operating system adds failure modes for no benefit.
Use a USB drive formatted as FAT32. Many board updaters cannot read exFAT or NTFS, and a drive the updater cannot see is a common and confusing first obstacle.
After it completes, re-enter the BIOS, confirm the version, and re-enable the memory profile.
Read the release notes before updating
Manufacturers publish notes for each version, and they are worth reading rather than skipping to the download.
Two things to look for. First, whether the update addresses something you are actually experiencing; if it does not, the case for applying it is weak. Second, whether it is marked as not reversible, which some are. Firmware that updates the processor microcode or security fuses frequently cannot be rolled back.
Also note whether the update resets settings or requires clearing CMOS afterwards, which the notes usually state and which affects how much reconfiguration you should expect.
What to expect afterwards
The first start after an update takes noticeably longer, often thirty seconds or more with a blank screen while the board retrains memory. This is normal and is not a failure; do not reset the machine during it.
Settings will be at defaults, which means the memory profile is off and the machine is running memory below its rating until you re-enable it. Fan curves, boot order and any other customisation will also need setting again.
If the machine does not start at all afterwards, clear CMOS first. That resolves most post-update problems, and it is the step to try before assuming the update failed.