Firmware updates for the Nintendo DSi are downloaded and installed right from the console's Internet connection. However, depending on the age of your hardware, not every firmware update may be best for every model. If you want to roll back your DSi and uninstall an update that you've downloaded, you can do so by performing a hard reset on the device. This turns everything back to the original factory settings.
Turn on your Nintendo DSi.
Open the "Settings" menu.
Select "Next" three times to advance to the fourth page of the "Settings" menu. Select the icon that reads "Format System Memory."
Select "Yes" to perform a hard reset on your Nintendo DSi. This will uninstall any updates you've downloaded and return the DSi to its original factory settings.