Upgrade SGH I337 S4 with 5.0.1 to at least 6.0.1 - HELP

[Android Central Question]

I have an SGH I337 S4 that has 5.0.1 on it and I need to upgrade this system. App upgrades are being hindered. Working through Kies and/or Smart Switch and the ordinary update options are not giving me an opportunity to do a true OS change (or I can't figure out how to do it). I could use a recommended path of travel. Thank you in advance.

 

[B. Diddy]

Welcome to Android Central! From what I can tell, the S4 never got above 5.0.1: https://www.sammobile.com/firmwares/galaxy-s4/SGH-I337M/. I'll move this to the S4 Rooting/ROMs forum, since you'd have to find a custom Marshmallow ROM and root the phone to install it, if this is important to you.