How do I downgrade the baseband?

You can't. Once updated, never downgradeable. Only very old iPhone 3G versions with the old bootloader 5.8 can be downgraded. If you have such an iPhone, you probably know it already. For the iPhone 4 you can install the baseband that Apple allows. Apple only allows the newest baseband (with a few exceptions during beta-tests), so there's also no way to downgrade. If you need an unlock, make sure you never upgrade your baseband.