How do I change green paint to grey?

First, you MUST paint over the green with a primer and then paint grey over the primer.

If you mean you have a tin of green paint and what to add something to it to get grey paint - it won't work. You will always end up with something muddy brown.

You can't. Well, you could, if you added enough white and black to obliterate the green tint. Depending how green it is, and if you had a gallon of green paint, you might have to use 10 gallons of white and half a gallon of black to get rid of it.