Have you done all the usual stuff? Like, have you lowered your brightness (or set it on automatic), turned off WiFi/GPS when not in use, increased the amount of time between syncing for certain apps, etc.
Also, if you go into your settings and then Batter & Data Manager, then select Battery Mode, there should be some options in there to get better battery life. Make sure you aren't on Performance Mode too.
Also, have you looked to see if any apps are showing up in your battery usage? You might have a program that is soaking up too much battery life.
Or, you battery could just be really old or used a lot and you might need a new one, depending on how old your phone is.