Where can i download iphone 4 6.1 firmware?

I've checked felixbruns.de but the iphone 4 6, 1 firmware doesn't work when i try installing with itunes

So where else can i download apple iphone 4 6.1 firmware?
thanks

Added (1). I'm not downgrading but upgrading from 5.1.1 to 6.1

You can't downgrade from iOS 6.1.3, unless you mean updating to it. Put your iPhone in DFU mode first, and then try.

You can't downgrade