How to replce a new battery for my iphone 3gs

Its really simple, you need a buy a replacement battery from an online vendor and also a repair kit (probably from ifixit, they are best in the business) and follow the video and instructions on how to replace the battery

http://www.ifixit.com/iPhone-Parts/iPhone-3G-Replacement-Battery/IF137-002

You can first check the battery health of your iPhone, which is a function equipped on an iPhone running iOS 11.3 or higher. Head to Settings > Battery > Battery Health to check your iPhone battery health. According to Apple, the closer your device to 100%, the better your battery status is. If your iPhone battery health drops lower than 80%, that means you need to change a new battery in Apple Store. You can contact Apple for battery replacement appointment. If the battery health is higher than 80%, and it still drains faster, you can read on this tutorial to improve your iPhone battery life: 12 Ways to Fix Fast Draining iPhone Battery