Galaxy S4 not updating, still on 4.2.2.

[Introvert4point0]

I've read several articles and forum posts about this and haven't really gotten a good answer to help me out. I traded this phone a couple weeks ago, its used. Works great and all but when i try to wi-fi update my software it goes through with it all, resets it, then says its was interrupted and i must wait 24 hours or 12 hours or something. Now this phone i think was rooted, when it resets a little robot guy comes on the screen and it says something about safestrap or something idk and asks to recover or continue. So if somebody could just give me step by step or a link to a clear step by step to fixing this so i can update my software i'd appreciate it greatly. I've never rooted or unrooted a phone or anything so i have no earthly idea what i'm doing and i dont want to ruin my phone. Thanks.