回答“I've just replaced the batter in my iPhone 6s, but the new one won't c”Run the new battery until it is completely drained. Then, try charging it up. If this doesn’t resolve the issue, then the replacement battery is probably faulty.
回答“What parts are recommended for an upgrade?”On the apple website, I think there is a way to install older versions of macOS. For your specific iMac, I’d put macOS High Sierra or Sierra, which I think should work with your iMac.