Lindsay, essentially what's going on here is this.
Anytime Apple releases a version of iOS it signs that version. Without getting complicated, all it means is that that version is the only one that Apple considers to be valid. iTunes and over the air updates for normal non-jailbroken iDevices can only restore or upgrade to a version of iOS that Apple is currently signing (valid).
When Apple releases a new version of iOS it sometimes gives a small window where it will keep signing both the old version and the new, making both of them valid at the same time. But inevitably, Apple will stop signing the old version making it invalid and thus you can only update or restore to the current (valid) version of iOS.
With a jailbroken device there are a number of ways to roll back, a lot of them involving software and procedures you are probably not familiar with. But your device would have had to have been jailbroken to take advantage of that. Since it's not, you're stuck where you are as TriJetHero said.
In versions of iOS before iOS 6, rolling back was more or less non-complicated. But Apple changed things with iOS 6 that affect the iPhone 4s and up quite differently now. So, now it's even harder for those who are jailbroken to roll back if they have an iPhone 5 or 4s.
I hope that gives you some idea of why you are being told you can't.
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I don't think or not sure if I can upgrade my OS on this computer anymore. (and don't have access to a newer OS right now)
I have a old blue tower that I have updated:
1.25 GHz PowerPC G4
1MB L3 cache
2GB Ram
That's not going to work. iOS 7 syncing requires iTunes 11.1. The max that any PowerMac can use is iTunes 10.6.3.
You are in a rock and a hard place. You'll need either to find a crappy PC that can run iTunes 11 or buy an Intel Mac.