how to update iPhone 4 to iOS 6 NOT iOS 7??

I have the iPhone 4 running on the iOS 4 system (ancient, I know) but it runs perfectly and I've never felt the need to change it. Now, with BBM on iOS, I need iOS 6 at least to run the app....

I really don't want to update to iOS 7 due to the terrible sluggishness that will come as a result. Is there any way I can update to iOS 6 instead of 7... ?? I'm desperate!