There are two tools for changing the current view:
This tool can be used to zoom in (left click), zoom out (right click), or zoom the whole canvas around a particular region (draw a rectangle).
You can also access the zoom functionality by holding Ctrl and scrolling the mouse wheel.
Using this tool you can scroll, or pan through the image. This is particularly useful for moving through various parts of a document when the zoom is set high.
This functionality is also usable from any other tool at any time: simply hold down the spacebar and then click and drag with the mouse.