To zoom in
Double-click with the left (primary) mouse button at the location to zoom in on. Alternatively, move the mouse pointer above this location and then press I. Or, better yet, if you have a mouse wheel you can use that to zoom in and out.
To zoom out
Double-click with the right (secondary) mouse button at the location to zoom out from, or use your mouse wheel if you have one.
If you have modified the mouse button behaviour assigned to the zoom operations in the Miscellaneous page of the Preferences window, use that method instead.