Mouse scroll up or down for zooming in or out, you can also click + or - in the Navigation bar.
Click on the Zoom into a region() button then click on the Map window and drag to create a rectangular area you want to zoom in and release to zoom.
The same can be acomplished by holding down the shift key and draging into the desired region.
You can zoom to a specified scale by selecting it from a dropdown menu in the bottom right corner of the Map window.
Click on the Full extent() button to zoom to maximum map extents.