You will probably find a You Tube demonstration of how to do it. But, I'd be wary -- I followed one of those on how to dismantle a iPod mini (though I have been fixing electronics for years) I managed to break at least two parts of the thing.
I'm afraid Apple products either seem to be designed as disposables or to deliberately obstruct DIY fixes.
I would guess that when they change the battery at the factory the inflated cost is partly due to the risk of having to replace your phone completely.
It's a nice phone. But battery life is a problem and the non-user-replaceable battery is just absurd.