Have an S4 with custom ROM. What do I need to know to update?

[drywater]

Hi there,
I just moved from Windows Phone to Android, so this stuff is pretty new to me. I have a Samsung Galaxy S4 that came from FreedomPop (Sprint). It had a bunch of FreedomPop stuff (custom rom?). I wanted to move to RingPlus (also Sprint) which I found some tutorials for updating (or upgrading?) the ROM. I successfully got it updated to 5.0.1 and also got the modem working. However, there is now an update to fix a "critical security" issue. I get something like:

"/system/<SOME>.apk" has unexpected contents.
E:Error in /tmp/update.zip

Reading into this I read that if I have a custom ROM there are work arounds. My problem is that I don't even know what my custom ROM is. How do I start to familiarize myself with this? I have no interest in customizing anything further, just would like to be able to install updates (or updated ROMs containing the updates) as necessary, without jacking up my only phone.

Where should I start reading?

Thanks!

-Stephen

 

[drywater]

I should mention I first used Samsung Kies to update the phone (not sure what the update was though, maybe the firmware?). This disabled my modem which was a simple fix using ODIN and a modem driver. Unfortunately I also don't recall what all that did, if it was a full ROM update or just the driver) but it did the trick. According to Kies I currently have OD2 but can upgrade my firmware to OH1. But, again, I'm not sure what that does and don't want to fool with the fiasco of a phone without wifi again.

FWIW, the phone is SPH-L720 on RingPlus (Sprint). If it helps to know more about the software/versions please let me know and I'll try to figure out where they are in the phone settings.

Thank you again!

-Stephen