Super SU Unroot Help topic

Hey guys,

I have a Galaxy s4 I9505 which has been used a bit already. It reached me with already installed Super SU app. Since my phone is already rooted, i wanted to unroot it and found an option on it but it didnt unroot it, rather kept saying please wait and then i gave up. I restarted my phone and now when i try to open it it says there is no SU binary installed.....

Now, how can I unroot this phone so that i can update it.

Thanks!