Militarily, North Korea still uses old soviet weapons that have not been updated to modern standards or not updated at all. They could invade South Korea but they have one of the most modern armies in Asia. Even if the North invaded the South the North would run out of supplies just like in 1950 at the Pusan Perimeter. Also their air force, if they even have one, is extremely pitiful and probably uses outdated MiG-21s and MiG-15s.
The only way they could win would be to use a nuclear weapon on their own land.