Your phone can't update past iOS 12.5.7. If you are asking why it can't be updated past iOS 12, it's because your phone was released almost 10 years ago, is too old, no longer supported and lacks the ram required to support the newer iOS updates. If you need a phone which can run iOS version higher than iOS 12, then its time to retire you very old iPhone and buy a more modern, newer iPhone which can run higher level iOS versions.
As to your battery question, batteries are consumable parts. If you've never replaced the battery in all these years, it would make complete sense your very old battery depletes quickly, because it likely went past its useful life some time ago.