If it's iron sights, some class that I took (Appleseed probably) told us to think about the rear sight as "our buddy" since it's closer to us. Move the rear sight in the same direction that you want your group to move "because you and your buddy want to do the same thing". Moving the front sight will move your group in the opposite direction than what you move it.
If the "buddy" thing doesn't work, just use your fingers. Make a V with the fingers of one hand (rear sights) and use your index finger on your other hand (front sight). Line them up like sights. If you move your V to the left, you have to bring your finger to the left to get back into the notch, moving your muzzle (and group) to the left.
If it has a scope, it should be marked on the turrets how to adjust it.