If it's an old one (most likely light gray and white or light blue), normally you should be able to completly erase the calc OS by removing a battery, then hold down Home, P, Enter and ON keys at the same time until you get a menu after the loading bar reached half. The following will cause all your programs, games and files to be lost! Then select an option to format the calculator including erasing the entire OS. Once done you need to reboot again then when asked, send a new OS.
If it's a newer Nspire, the one with a touchpad instead of clickpad, you cannot downgrade lower than OS 2.0.