See the following Apple articles ⤵️
Charge the iPhone battery
Charge the iPhone battery – Apple Support (UK)
Show the iPhone battery percentage
Show the iPhone battery percentage – Apple Support (UK)
iPhone 6 is an outdated iPhone (9 september 2014) -> nearly 8 years old, which also affects the battery life.
The hardware remains old, so does the battery.
Not sure if it is still possible to have the battery replaced, check here:
iPhone Battery Repair & Replacement - Apple Support
The battery can be replaced by Apple or an Apple Authorized Service Provider.
If it is still possible and if you are going to replace the battery, read this Apple article:
Get your iPhone, iPad, or iPod touch ready for service
Get your iPhone, iPad, or iPod touch ready for service - Apple Support
Click here for more support and assistance.
or -> call 1-800-MY-APPLE