Updating to Windows 10 might have disabled/removed the Samsung Settings application which enables the function keys. In your case you should try to download SW Update from Samsung's website and install Samsung Settings and any other necessary drivers from there. If you can't get this to work, I suggest you refer to the latter pages of the 2013 Series 7 chronos / Ativ Book 8 15" owner's lounge (NP770Z5E / NP780Z5E / NP870Z5E / NP880Z5E) thread on NotebookReview.
Did you upgrade from w7 or w8.1? From w7 you need to install the w8.1 Samsung settings and then choose comptability mode 8.1 in w10. Don't use the easy settings from w7. This worked for me. Only issue I have is the numeric keyboard that stops functioning after it is booted.