Step 1: You can't.
Despite the many theories or conspiracies you may have heard, as of today, it is simply not possible. You cannot go back and contradict the I Told You So's or resume the Never Again's. The way we live life now is almost like a TV without a recorder. We can photograph or videotape certain parts that we want to remember, but there is no real way to go back and replay the whole episode. Whether you are for or against this obscure phenomena, it is probably best suited in an underground lab of a mad scientist and here's why.
Life just happens. It happens without the push of a button or the plug in to an outlet. If you asked anyone to sit down and write a list of all the times, recent or not, that they haven't been satisfied with the way situations occur unto them, most of them would be long and harshly internal. These unsatisfiable times that we go through, may they seem often or not, all show us a layer of not what we are destined to become, but what we already are. For example, always stubbing your toe on the last step of the first floor of the train station might mean you're a klutz. However, you don't let that define you, you merely buy a sturdier pair of shoes and go get your eyesight checked. Occasionally failing to give a unnerved speech in your communications class does not define you. One day you will realize that these people you are speaking in front of are purely people that you may never see again and you are doing this to better your career and communication skills and you are just speaking, which is something you do all the time. These people will not judge you because you are not alone in speaking one after the other and your idea is probably brilliant and you are indeed confident, of which you have always secretly been.
Learning about our secret confidence and clumsy nativeness are just two layers of a potentially infinite layered person. The people you run into and the scrapes on your knees are not accidental, for they are intentional in revealing our reflection in a foggy mapped out mirror. Sometimes people abandon you when they are weak, because an earthquake could replenish when they are strong. Sometimes people are selfish, because they don't realize the opportunity to be otherwise. People live because it is inevitable to until it's not. Without the constant progressive happening of what we call life, there would not be a real timeline of what we are. Having the controls or remotes of this timeline, there is no real promise of a necessary future. Which is undeniably more important than the past. You cannot go back and contradict the I Told You So's or resume the Never Again's.You cannot go back and make a better grade or say sorry to the boy you fought with in 6th grade. You just have to rely on the promise of the future and that will be time travel enough.