In Google Maps built-in App, I click to map to move and get detailed map[zoon in], well, how zoom out?

Take your thumb and index finger, spread them apart and place them on the screen. Now bring your thumb and index finger together by sliding them on the screen and the image will zoom out. It works exactly the opposite way too! Place your thumb and index finger together and slide them apart on the screen and the image will zoom in. This method works with any graphic on the iPad, iPhone and iPod Touch.

There is all kinds of neat stuff in the User's Manual!