I have NUC8i5BEK. I updated BIOS 51 -> 56 from within Windows 10. Update showed that it was sucessful, however it is broken. When turned on, display just showed pattern of random colors. I reverted BIOS to version 51 by emergency method. So far, so good, but when in Windows, it shows notification "you need to restart to finish installing 56". As soon as I restart, it "reverts" to this newer broken update and I need to again manually install older version. But Windows still keeps showing that I need to restart to install 56. How to remove that pending installatin in Windows? Thanks.
I had no issues updating to this BIOS release, either during or after. I suggest that you try the update again - but this time using the F7 method.