Root Recently left AT&T. Recieved unlock code from them. How do I install non-carrier Android OS?

I'd like to install Android 4.4.4 or Android 5, on my unlocked and rooted S4 Active... I'm not a tech genius. Although I think that there are retail operating system ROMS for Android. Meaning a Google-retail, non-carrier version. Is there? Where do I find or download it please?