Battery on mp3 player dies alot

Hi. I have a , uh, somewhat annoying, problem with my Sansa Fuze 8Gb Mp3 player. i know they're advertised as having a 12-hour runtime, but on mine the battery goes down faster than i charge it. I barely play videos on it cause i know it runs the battery down a lot faster than playing just music. i would like feedback on this, as well as step by step instructions for replacing the battery with one with longer runtime.
Please & Thank You!