The best thing to do if you have problems after updating your iOS is to restore your device with your backup. If you still have problems, restore as new, i.e. without your backup.
A simple reset might work as well... press the home and sleep (on/off) buttons until you see the Apple logo, ignoring the slider. Takes about 5-15 secs of button holding and you won't lose any data or settings.
There is no legit way at all to downgrade.
Why not try restart, reset, restore to correct your issues?