How can I update an iPhone 4 running on iOS 5 to iOS 6, while iOS7 is available?

How can I update an iPhone 4 running on iOS 5 to iOS 6?

An update to iOS7 is available through the default update process via iTunes.

But as the device is quite old and rumors have it, that there will be severe performance impacts.

Therefor I don't want to upgrade to iOS 7, but just to iOS 6.

Is it possible?

Where can I download the update package?

Thanks in advance!