Normally you will not have to restart the Apple Watch app. This usually indicates communications issues between the Watch and the iPhone it is paired with.
To achieve a restart of eKeypad on the Watch you will actually need to restart all apps. There is no way to stop a single application.
- Press and hold the side button. This is not the Digital Crown, but the oblong button beside it.
- After a few seconds the Apple Watch will show a shut down screen.
- Release the side button, then press and hold it once more for several seconds.
- The Apple Watch will automatically return to eKeypad, close it automatically, and open the home screen.