IPhone 6 keeps saying it’s dead and I know it’s not.

For the past year or two my iPhone 6 has been having battery problems. The battery will suddenly go from being full to being at 10% even when im not using the phone. and today it’s been turning off and saying that it’s dead, I get the black screen with the battery icon. My phone has been plugged into a Charger all day and just turns on and dies when it wants to. When I go to the battery section in my setting it does say that my battery may need serviced so I’m pretty sure there is something wrong with my battery but I don’t know like what i would need to pay to get it fixed and all that kind of stuff. If there’s an easy way to help this problem that’ll help a lot.