Tap and hold to have your tank move to that location. However, because you finger will cover part of the screen you may miss incoming attacks. You can reduce this issue by constantly moving your finger around to keep your tank moving all the time. Try doing this near the bottom of the screen, as the alien ships usually appear from the top or the sides. However, if you accidently tap the ad at this location, it will obscure your screen and cost you a life. To avoid this, keep your finger on the screen to prevent a second tap.