Update from iOS 6 to 7, but not to iOS 8?
As I had mentioned in my comment, Apple is no longer signing iOS 7 which means you can either update to iOS 8 or stay on iOS 6. Usually after an iOS update release, the previous iOS version is supported for a little while and then the signing stops. Right now, Apple signs iOS 8.1.
If you want to know what firmwares are being signed by Apple, there is a neat little website which is updated with the information - http://api.ineal.me/tss/status