I would preheat the oven to about 250-275. Put the nachos in there for like 10 minutes. Keep an eye on them though. You don't need to cook them again, just warm them up and melt the cheese.
I wouldn't use a microwave either, it will make your chips soft.
I dident know nachos were heated in the first place?how should I reheat an arby's sandwich?
Just heat the cheese in a pan or in the microwave.