Can't get Nvidia graphics to work

[jack169]

I have a laptop with an Nvidia GeForce 8400m GT. As of now, all of the windows lag when I drag them or scroll. The desktop resolution also doesn't match the display's. I went to "Additional Drivers" and installed the 340.96 binary, then rebooted, but everything stays the same. If I go the X Server Settings, it doesn't show any kind of display or graphics options. It just has one tab with a few other options like "Display Status Bar". I've tried to manually download a driver from the Nvidia website and and install that, but it gives me
The kernel header file

 '/lib/modules/4.2.0-30-generic/build/include/linux/version.h' does
         not exist.  The most likely reason for this is that the kernel source files in '/lib/modules/4.2.0-30-generic/build' have not been
         configured.

Can anyone help me? I just want to get the drivers up and running so that the resolution is correct and the windows don't lag.