Going back to an iPhone from HTC Windows 8x?

Okay, so I had an iPhone 4 and my mothers phone broke so we decided that she could have my iPhone and I got the HTC Windows 8x. I've had it for a few weeks but don't like it as much as I liked my iPhone. So I found someone who will trade me their iPhone for my HTC and I was wondering if I could transfer my sim card myself and it will work instead of going to the store and having them charge me. What do I have to do to get it to work?

Added (1). I found someone on Craigslist who wants to trade, what I want to know is if I put my sim card into the iphone, will it work?

EBay/craigslist/amazon is what you need.

I've heard that from alot of people… Moved to a different phone from the iphone because they were bored or whatever reason… And now are stuck with it and trying to get back to an iphone again.

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