Here you will find instruction how to replace the CMOS battery:CMOS Battery on the Intel® NUC . And yes, it is quite complicated, since you need to remove the board from the case - the battery is located on the board upper site. Following the Intel's instructions, you can do this by your self (without waiting for service center).
You need a coin-cell battery (CR2032). You need battery with pig-tile and connector. This is example of such battery: 1 CMOS Battery CR2032 For HP Pavilion DV6000 DV6100 DV6200 DV6300 DV6500 DV9000 | eBay . There is a lot other suppliers online.
I'm attaching an image of the board side with the CMOS battery.
Hope this will help
NUC6i7KYK_CmosBattery.jpg 98.9 K
How to do that without damage?
I remove all 7 screws from fan and motherboard, but something else holds them. I only can lift a bit fan from one side.
What should be removed earlier, fan or motherboard? I see some wires at top side (under face plate):
What must i do with them? Wifi-module are soldered.
Don't remove top cover. As I said, gently pry along front edge of board (to free it from connector openings). The board and heatsink/blower unit flip up as a set. Do not disconnect antenna leads; there should be enough play in the leads for you to access and unplug the battery from the board without doing so - if you are careful.
To be safe, wait 15 minutes before plugging the new battery in. Make sure battery is struck to board where old one was. Then, gently push the board and heatsink/blower unit back down into place and then reinsert the 7 screws. Put the bottom plate back on and you are done.
I am serious, don't remove more than the 7 screws and don't disconnect the antenna leads. If you do, you are multiplying the work necessary.
Hope this helps,