How to update the software of Samsung Galaxy s4 even if rooted (currently unrooted via SuperSU)?

[AC Question]

Well I have a Samsung Galaxy s4, I've rooted it since I've got it, however I cannot afford to OTA update my phone since it's been rooted. But now I've unrooted it via "SuperSU one click unroot" but it still gives me that message "your device has been modified, you cannot update your software" although I am unrooted. But maybe it's still there? I don't know. So afterwards I came to a near by Samsung Help Center but they said that I'm pretty much doomed (that I can no longer update my software anymore) although I believe that there's still another way to update my s4. But I don't know how, I've tried reading other people's solutions, however I'm doubting that I'll never get it since it's not that clear. So please tell me that there's still another way to update my software via "not OTA" but can still update my s4 to the latest os.