From the Arrange window's local menus, choose Edit > Toggle Selection (Shift-T).
The selection range inverts, or toggles, so that all unselected Regions are selected, and selected ones are deselected. The Toggle Selection command comes in handy if you have to select all but a few Regions in the Arrange area, because it's often easier to first select the few Regions and then toggle the selection.
More Info
You might have noticed that the Edit menu contains several selection options. There are too many here to go over individually, but they all have specific functions that are immediately identifiable by their names. Pay particular attention to Select Inside Locators (Shift-I), which is a good way to select all the Regions that make up a particular verse or chorus, and Select Muted Regions (Shift-M), which offers a great way to select all muted Regions for deletion.
