How to fix my iphone battery?

My iphone dies at 50 percent so it obviously doesn't last the while day, hiw can I fix that myself? I got it in December so it's not even that old

Take it to apple store don't touch it!

It is still under warranty, Apple will fix it for you at no cost.

It should be under warranty. Take it back. Once it is fixed ask if there are any tricks you should use like always running it way down or never running it way down. You either have a defective battery/charging system or the battery has devloped charge memory from constantly being recharged when it is not run down. That is less common than with older batteries but it does still happen.
If they try to tell you it is not covered ask to speak to a supervisor and even the supervisor's supervisor!