How you restart iphone and switch to different carrier?

Ok we figure apple store they reset the iphone we figure out you need verizon to get it unlocked but after what do you do you go to AT&T then after you go on the itune to set it up how you restart process

If it's an iphone that takes sim card like T-mobile AT&T you just switch out the sim card from the phone considering that the phone is already unlock. For companies like verizon, you have to go to there store so they will activate the phone on there network or either call them also they don't use sim cards