My friend has an old iPhone 3G and I wanted to upgrade it to iOS 5 because I had read that the performance is actually better with iOS 5 on that phone in spite of the difference in processor power.

The problem is that now that iOS 6 is out, iTunes only gives me the option to upgrade to that. I am a little more leery of iOS 6 knowing that it was launched mostly for the iPhone 5 and since that phone has a way faster processor, I would assume that performance would be an issue on the iPhone 3G.

How can I, if possible, upgrade just to iOS 5?

Note: I would like to avoid having to jailbreak the phone by all means possible. (It's not mine and I don't want to take the risk with a phone that doesn't belong to me.)

EDIT: If anyone knows of a good site to get help on questions like this let me know. Apparently you're allowed to ask questions about iPhones here, just not this. Which is completely utter asinine, but oh well. Anyone with helpful ideas, post in the comments please.