Generally lithium-ion batteries have been used in all mobile computing services, inevitably become less efficient at storing charge as they age. After a few years, the battery life of any smartphone, tablet or laptop will drain faster than what it was brand new. A rough estimate would be that a battery's maximum capacity drains by 10 percent each year. The iPhone is no exception but unlike many phones, the iPhone was designed by Apple that can't be opened easily and let you replace the battery with a new one.There are reasons for this restrictions. It's why there are no ugly gaps in the iPhone's chassis and it reduces the chance of dirt or moisture getting inside and cause other major problems. Although this also means that you can't buy simply a new one and install it yourself. In that case either you need to look for professional yet certified iPhone service center or look for the new phone.
the major reason for accessing the services of the certified service center is that they only install genuine products and the service is free if your phone is within its one-year warranty. In case, your phone isn't covered by any form of warranty, the technicians can still replace the battery it would be chargeable which includes extra service fees.
Furthermore, the other benefit of going to certified service centers is that they will provide you instant and quick iPhone battery replacement services by a certified engineer who have been trained to handle such problems and able to repair it without leaving any scope for costly errors. Getting your iPhone battery replaced by the local certified professionals can help you get the services at a low price as compared to Apple store and enables you to bring your phone back in right working position.