How to reinstall previous iOS version on iPhone or iPod touch
This might be not possible, depending on what device you have. Because Apple has stopped signing 4.2.1, unless you have backed up your SHSH, there is no way back on iPhone 3GS or 4. 

If you have jailbroken before, Cydia might have backed up your SHSH. Then you should use Cydia's server to perform a restore.