• All Bioses update files for NUCs are available on Download Center.

Follow this FAQ to download:

[NUC] Where Can I Download the Bioses File?

• Update instructions are also available in PDF format.

Bioses Update Instructions for ASUS® NUCs with Aptio V UEFI Firmware  
 

• Once the update process has started, DO NOT POWER DOWN YOUR SYSTEM before the update is complete. The update process will take up to 3 minutes. 
• Downgrading the Bioses to an earlier version isn’t recommended and might not be supported. An earlier Bioses version might not offer support for the latest processors, bug fixes, critical security updates, or support the latest board revisions currently being manufactured. If you’re trying to downgrade to a previous Bioses version and its Intel® ME Firmware version is older than that currently on the motherboard, then the downgrade is NOT supported.
• There may be a situation where the Bioses doesn’t see your USB flash drive during an update or recovery. If this happens, try to reformat the USB flash drive as FAT32 format, but de-select Quick Format. Then try the process again. You can find out more information in 
• If a Bioses update process is interrupted, your computer may not function properly. We recommend the process be done in an environment with a steady power supply (preferably with a UPS).
• Before you start, record all Bioses settings that have been changed from default, so they can be restored after completing the Bioses update if needed.