My iPhone 7 Plus battery's maximum capacity is 86%

You shouldn’t need to worry until it is about 80%, anything above this is deemed optimal and your phone should run as normal.

This should explain it all a bit more for you: https://support.apple.com/en-us/HT208387

Typically you don’t change it unless it says to Service or is close to 70% capacity. https://support.apple.com/en-us/HT208387

There isn’t any free replacement programs either, but it is pretty cheap thru Apple. https://support.apple.com/iphone/repair/service/battery-power