Battery Health

I believe Apple recommends the replacement of the battery once it hits 80%. However, sometimes you can go beyond that, as in the case of AppleFan3421. Read the info under battery health closely. If it says the battery is no longer able to provide sufficient power for peak performance, it needs to be replaced. I had one at 85% that had to be changed for that reason. So, it varies a bit on when to replace the battery. My current phone is 9 months old, and I have 99% battery health left, but I've read where some are in the lower 90s after the same amount of time. Some batteries last longer than others, and how often and how heavily you use your phone will affect this as well. Your best bet is to take it into the Apple Store and let them run a diagnostic on the phone and replace the battery if necessary.

Do you have Apple Care? If not, you can go to any repair shop or Apple Store and they will change the battery for you (you have to pay of course). Otherwise, you could just stick with the battery. My iPhone 7 has a health of 78%, and I am still using it.