Samsung S5 update

[AC Question]

I have a S5 from ATT that I am using ion the cricket network. I am trying to upgrade the software. I am stuck on the Android 4.4.2 Any tips on how to push the software into the phone? Every time I check for upgrade says up today. Even Kies says that! But I read the the S5 is 6.0 now. My apps are not running as well also. I now I had a Fascinate I rooted and push more up to date software into it until that quite making updates.

Thanks
Randy