how to replace battery on iphone 7plus
How can I replace battery on my iphone 7plus all by myself?
iPhone 7 Plus, iOS 11.2.6
You would need to buy tools to open it, a third party battery and buy a guide to walk you through it. You will not be able to buy an Apple battery as they do not sell parts and do not consider the iPhone to be user serviceablye. You will also void any warranty you have and not be able to get future support from Apple.
You would be far better served getting it serviced by Apple or an Apple Authorized Service Provider. You really should not need a battery replacement yet.