They would be able to do some large-scale damage to Seoul in South Korea and maybe another large city, but after their initial attack, they will be wiped out.
Their army is very outdated and their resources and food limited, at most they have stockpiles to last for maybe 3 months, but South Korean, Japanese, US and Western forces would wipe them out after that.
If China came into the mix, they wouldn't stand a chance either.