I've have upgraded BIOS of my NUC6i5SYH to ver 0065 without any problem. The NUC performance have not been degraded. Works good. For some reason (I don't know why) Intel has removed ver 0065 from download page leaving the 0063 as the last version. Follow the following steps to recovery your BIOS to ver 0063 from Power Button Menu:
1. Insert USB with SY0063.bio file into USB slot.
2. Press on Power Switch and hold it. You should release the Power Button soon as the power led changes color from blue to amber. NUC will reboot into Power Button Menu.
3. With the USB containing the latest bios file, press F4 and the recovery shall start.
Hope this helps
I force the NUC recover to 0063 by pull out the BIOS jumper on mainboard.
I also tried the method as you said: Press on Power Switch and hold it. It does not work. The NUC power on for a few seconds, and then shutdown itself.
Now, after downgrade, NUC looks okay.
Anyway, thanks for your help.
I will presume that the BIOS was withdrawn because of (take your pick for which source to read it from):
Any way, I've updated my NUC using recovery method and have not observed any problems. I'll wait what will happen next.
I upgraded my sister machine to 65 thinking I was helping, so far no issues. will probably leave it since she lives quite far.