If I get an iPhone with bad esn how can I make it good esn?

My daughter just bought an iphone4s with bad esn? How can I fix the bad esn?

Can't do it. Your daughter got taken.

You can't "fix" it.

No it can't be fix. The previous owner have report the phone lost, stolen, or just not paid the phone bill. So the phone is now bad esn and can't reactivate to the original carrier. There's no way to fix it, but you can unlock it because iPhone 4s has a Sim Slot. You need to buy a sim call the "gpp sim".It's around $20 on ebay. Here's a video on how to unlock the phone with the gpp sim.

You will be able to use any Sim card.