Default [Q] How to update Att&t S4 I to kitkat or install any working rom with safe strap? ??

I have a sgh I337 s4 which is still on android 4.2.2 I want to update it a new os or even a custom rom for better performance. I've been trying for weeks every time I try to update it gives me the whole " update interruption try again in 24H" ... I have tried to install many custom recoveries only one that worked was safe strap I am a newbie looking to get better performance out his phone and to use my galaxy gear please help ... my phone is of the "MF3" build ... I've tried do a factory reset but doesn't work every time it goes to "downloading target do not turn off" and remains that way for hours!!!! HELP!!!