If your NUC is NUC6i5SYK it has i5-6260U (skylake) CPU installed. You have not mentioned what operating system are you using.
Download the graphic driver here and install it manually (for Windows 10 64 bit): Download Intel® HD Graphics Driver for Windows® 10 for Intel® NUC Kits NUC6i[x]SY
or Windows 7 and Windows 8.1: Download Intel® HD Graphics Driver for Windows 7* and 8.1* for the Intel® NUC Kits NUC6i[x]SY
However if you have Intel® Core™ i5-5250U CPU installed, your NUC is NUC5i5RYx series and the drivers you can download here: Downloads for Intel® NUC Kit NUC5i5RYK
Hope this helps