with the phone on, press and hold the power button until the shutdown menu comes up, hit recovery. The phone will reboot into recovery mode and then tap wipe and cache, swipe where it says to swipe to clear and then go back and do the same but select dalvik and swipe again to clear that, then hit the back button until you see the reboot option and choose system to reboot.
I followed your directions and I now can change and set my ringtones.
![Smile :) :)](/styles/smilies/google/smilie.png)