...iOS 3.1.3? So i bought an iPhone off my friend and i've been messing with all day...I Got home and Fully updated it and now it locked on me and said that i need a Sim card or something. Sucks that i finally discovered how to put my Tmobile sim card into it and now that i have the iPhone actually Saying Tmobile on the top left, i screw it up...Can anyone help me figure out how to DOwngrade it back to Version 3.1.3? or anything lower?