how do I upgrade my Samsung Galaxy S5 kit Kat 4.2.2 to Android 5.0 or better

Hello all I'm new to the forum info not know how to upgrade my stock Gs5 from kit Kat to Android 5.0 or better. My device is a stock AT&T smg900a I would love to put cyanogenmod or some different kind of Android os. Any suggestions would be great.