dear apple
i have a iphone 4s and im am using o2 but i have changed it to three but it says sim not valid what do i do
apple is not here, this is a user to user forum.
Your iPhone 4s is not unlocked, it can only work with O2. You would need to request O2 unlock the iPhone so it can be used with sim cards from other carriers.
Only O2 can do this. Neither Apple nor Three can.