Galaxy S4 How to unroot?

I rooted my AT&T Galaxy S4 running Android 5.0.1 using kingroot and it worked great. I proceeded to try to install a custom ROM, which failed and I never even overwrote the stock ROM. So I am currently using the stock ROM. I recently got an update from AT&T for the stagefright patch, but every time I try to install the update, it puts the device into a secure boot download screen. I have to reboot the device to get back to Android. I also tried factory reset but it also causes the secure download screen to show up. I want to trade in to upgrade soon and I need to know how to fix this. Thanks!