From the OpenSprinkler user manual:
Reset: on OpenSprinkler 2.0 and prior versions, holding B2 while starting the controller provides a short-cut to the Reset option. Select yes and then hold B3, a hardware reset will be triggered, and the controller will recover to default settings.
From OpenSprinkler 2.1, holding B2 while starting the controller will enter bootloader mode, which allows you to flash the microcontroller with a new firmware. To enter Reset as above, you can use the Reset option.
Also, within the app you can backup the config without the password. Import it after reset.