You can zoom in and out on Google Maps by using the plus and minus signs in the bottom right of the screen or dragging the slider up or down. Alternately, you may use the plus and minus keys on your keyboard.
Once you have added the API, you must first turn the default controls off. This is done by setting Maps disableDefaultUI property to "true." You can find this property in the map options object. Once you have done this, you can alter several zoom options, found within the "zoomControlOptions" field. These include changing the style of the zoom slider by adding either "style:google.maps.ZoomControlStyle.SMALL" or "style:google.maps.ZoomControlStyle.LARGE" into the zoom control options field.
The first option removes the slider and leaves only a plus and minus button, while the second leaves the slider in place. You may also add DEFAULT in place of small or large, which uses whichever zoom control is best for your device and map size.