Question

How did the separate of kingdoms of egypt unite??

Answer 1

At this time the capitol of Upper Egypt was known a Nekhen while the capitol of Lower Egypt was Nekheb later called Heiroconopolis (the falcon city). 
After many years of warfare it was the warrior-king Menes (c3000BCE) whose armies finally defeated the armies of the southern nomes or city states, that are collectively known to us as Upper Egypt. 
hope this helps