In the iOS Simulator, press iOS Simulator in the menu at the top of the screen, and press ‘Reset Content and Settings…‘. This will clear the entire simulator.
Or, on the home screen of the simulator, uninstall the app in the same way you would on a physical device, by pressing and holding on the application icon until an ‘x‘ appears in the top left, and press the ‘x’, and select ‘Delete‘ from the alert view. This will clear all data associated with a single app.
Then do a clean, build and run.
Edit:
As of Xcode 11+ Simulator, this is under Device > Erase All Content and Settings