I want to change my battery how can i
How can i change my battery
iPhone 8 Plus
The battery can be replaced by Apple or an Apple Authorized Service Provider.
See this Apple article:
https://support.apple.com/iphone/repair/service/battery-power
If you are going to replace the battery, read this Apple article:
Get your iPhone, iPad, or iPod touch ready for service
https://support.apple.com/en-us/HT201557