I will give answer from technical perspective.
First off there are many guidance systems, some do not require any connection to any control so hijacking communications to the missile would only allow the hijacker to receive data that is being used for tests which probably isn't sensitive.
Secondly, if this is some American trying to strategies an attack on North Korea, if the missile is controlled for some reason from the ground then it is limited considering the speed that it is traveling at. Large objects traveling at high speeds are not easy to turn around considering the amount of momentum they have according to Newton's first law, the original launch angle would play a big part in where the missile actually ends up.
Thirdly control of the missile would likely be very secure and be done through preprogramming, anything else would be done over encrypted channels using some sort of protocol and would likely be turned off until the actual testing, so to be able to decrypt or reverse engineer the system would take a lot of work before the actual launch and North Korea's launches seem to be catching the world by surprise so I doubt they would be able to reverse engineer the communication system.
Lastly, it's a test missile not an armed ICBM, there isn't too much in it, it is not capable of too much damage and is probably tested over an ocean or test site anyway, to be able to change the direction to make use of it is not realistic considering the small blast radius, amount of technical data needed to hack something like this if it's possible, limited time to do something like this and the fact that turning around something with so much momentum would be difficult because of inertia.
Although if something is done while the unit is being built there are ways but that is very difficult because it would require some sort of espionage long before the actual test.