How to upgrade my iphone 4 from ios 5.1 to 6 now that ios 7 is out?

I have not upgraded my ios in a while. I have heard ios 7 slows the iphone 4 down so it may not be a good option for me. I would like to upgrade to ios 6 but don't know how at this point.

If you ever jailbroke your iPhone and used a software named TinyUmbrella to store your signatures, they might have iOS 6 signatures for your automatically requested otherwise, you can't install iOS 6.

Apple does not allow old firmware restores, so bad luck but you can only install iOs 7 in that case