Welcome to Android Central! You could try powering off (as long as it hasn't actually started the update process) and then boot into Recovery to wipe the cache partition. Not sure if this will clear out the downloaded update, but it won't hurt to try. http://support.t-mobile.com/docs/DOC-5744
Be aware that on many phones, you will ultimately be forced to update the phone--you can only defer so many times. The only way to permanently avoid the update may be to root the phone and disable the system updater.