Help Upgrade Android OS on Galaxy S4

[Android Question]

I have a Straight Talk (ATT) Galaxy S4 Model number SM-S975L. The operating system is Android 4.3. Can I upgrade this device with a later version OS? If so, which OS and where do I find it. Can you provide link to instructions?

Also...is there a way to unlock this phone from ATT? StraightTalk tells me it can not be done and are only offering me a small rebate for the phone. If possible I would like to have it unlocked to switch to Verizon.

 

[Jfalls63]

Straight Talk usually does not update their phones but there are a few custom Roms available
http://forum.xda-developers.com/galaxy-s4/general/guide-sm-s975l-twrp-4-4-2-custom-roms-t3009757
Straight Talk unlocking policy can be found here, section 6
https://www.straighttalk.com/wps/portal/home/h/legal/terms-and-conditions

 

[kate]

Also...is there a way to unlock this phone from ATT?... If possible I would like to have it unlocked to switch to Verizon.

It's unlikely Verizon will activate a Galaxy S4 from ATT/ST, but you can check here:
https://www.verizonwireless.com/bring-your-own-device/