So what you are trying to say is that Apple has a giant masterplan to cause battery drain on older phones? ok.
I don't own an iphone, but I would suggest to wipe and reflash and replace battery
I don't own an iphone, but I would suggest to wipe and reflash and replace battery
That's my point.
You can't replace any battery. Apple does not give you access to the battery.
Reflash how? I don't see how you can do that with Apple.
Looking on Amazon, you could buy a replacement battery+tools for about the same price.
I followed an ifixit.com guide (like this: http://www.ifixit.com/Guide/Replacing+iPhone+3GS+Battery/154...) which, combined with the video, was very easy to follow.
Alternatively, you could probably find somewhere to do it for you for maybe $50 or so (still cheaper than Apple), but I doubt they'd use anything more than the cheap stuff you find on Amazon.
Still, it's worth following the other suggestions first and seeing if you can downgrade the iOS version.