Welcome to the forums! Are you referring to a software update you just downloaded? I believe the only way to reverse this is to factory reset your phone back to its out-of-the-box operating system. Just make sure that all of your important photos, data, information, etc are backed up before you do the factory reset.
Even a factory reset won't revert back to the old version of Android. To do that you have to flash the older version. We'd have to know the type of phone to even lead you to sites that explain how, because each manufacturer does it differently.