Why won't my iPhone 4S update to ios 7? - 1

I bought it from someone on Craigslist it's AT&T unlocked using on t-mobile what could be the problem? I checked a few times it said 6.1.3 is the newest but my friends are updating to ios 7 fine

The servers are overloaded. Wait a few hours!

My iphone 4S is taking 4days (so far!) to download the 7.1 update. What's wrong?