[Q] Help, I'm 15 and want to unroot and restore to factory settings topic

Hi, i have note 3 sm n9005 or something and rooted my phone for some stupid reason, its still on stock the only thing that i think happened was i have supersu, i paniced and factory reset, now supersu has a error thing with something to do with binary settings or something. All i want to do it fully reset my phone, if its any help im british with EE... does flashing supersu trip knox counter? Am i actually rooted for if i were to flash stock? Will that trip it? Iv knly had phone few months and im scared... if i flash stock can i then update legally coz i cant update because the device is modified? I used odin... can someone help please, i know nothing and yes, i should look into what im doing more before hand. Can you explain everything and what to do please? Every little thing, please and without tripping knox and as if it was out the box new. Please