I pulled my Samsung 850 250GB SSD from a NUC6i5SYK I RMA'd.
I have Win-10 OS ad Apps on the old m.2 board.
Can I just install it in the new NUC. If not, how do I reformat/erase the M.2 SSD?
I did exactly that with an m.2 950 pro. I just stuck it into my 2nd NUC after the first one failed. It had Windows 10 on it plus some apps.
The only precaution I took is that I had read somewhere that on first boot you should not be connected to the Internet, to prevent Windows update from messing things up. So I disabled my router's wifi, just in case; then re-enabled it on subsequent boots.
The new NUC booted up fine, and has been working fine since then. The only issue was Windows re-activation. If you have a retail copy, it should not be a problem.