A Nintendo DSi can be reset in a few ways. One way it to remove the battery from the back, another way is to go into the 'settings' and restore the factory settings.
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1. Click "I forgot"
You can downgrade at this time, you can only upgrade
Remove the battery for about 10 seconds. Then put it back in, and your DS/DSLite/DSi has been reset.
Find the part that shows a picture of the Nintendo DSi Settings> Parental Controls and select forgot password, forgot secret question, now type the enquiry number.
New firmwares permanently overwrite each other, so even a factory reset doesn't remove them, there's no real need to though.