Go to the connecting bridge between the Jet and the Terminal. Go outside on those metal stairs and get on the flat (top) part of the railing. Run and jump onto a small ledge on the side of the Terminal. Follow the ledge left around that corner and you can get in the window upstairs which should be quite unexpected by the enemy team.

Obviously a stealthier way would be to come up from the bottom of those stairs, unseen, and then jump from the top of the stairs' railing to the ledge.