How do I reset the Simulator in Xcode 9

Xcode 9.4

You need to click on Simulator first. Hardware > Erase All Content and Settings. This will reset the simulator. Wait for a minute for the Simulator to rebuild.

Xcode 11.4

The simulator app no longer has the Hardware menu. It is now the Device menu.

Device > Erase All Content and Settings

I found it. It is now under "Hardware -> Erase All Content and Settings"

Edit: In Xcode 11 it is now under "Device -> Erase All Content and Settings"