Actually, the hard part about replacing the battery has already been done - opening the phone.
Nothing to lose but the cost of a replacement battery. Now is the time to back up important photos and other data if you haven't already done so.
If you decide to attempt repair, be careful to not cut any cables while opening the phone.
The phone is an old iPhone 6 which we only use for background sound at night in the bedroom. It’s a toss. Already ordered a “new” old phone replacement.
We could have just bought a MP3 player, but I wanted a functional phone in case of emergency.