IPhone 4S battery stuck at 90% for 2 days?

When I charge it, it doesn't change, and when I don't charge it, it still doesn't change. I haven't charged it all day, so would it technically be at like 40% right now? How do I fix it?

If it shows it as charging but the percentage stays the same it may just need to be calibrated. Do that by letting it drain until it shuts off and then charge it until it goes to 100%