I think the only "person" who can tell you exactly* what you need to do is the Akashic Librarian , she/he/it have a lot of people who turn up and want to rectify the past (mostly people who aren't even aware they are projecting and their immense grief or loss or an intense urge to rectify the past inadvertently leads them to the Akashic Librarian).

For instance a grieving mother who recently lost her teenage son , goes to bed , hugging onto her grief and projects with the urge to have done better in the past.
The grief severs as resolve and the urge to rectify serves as her guide towards the AR-librarian , who then asses the situation and accordingly guides the mother to the relevant part of the library.
What happens next isn't known , some people wake up with the all the exact directions to time travel and past-possession but dismiss them as bunk or "weird dreams" , also often times when these people share their information with their significant others or whomever they tell it of to , they are told these are their "escape mechanisms for coping with the grief" .
What happens to the people when they are in the relevant part of the library depends mostly on how strongly the person involved was meant to have this experience and how genuinely they qualify for a "do-over"

There are innumerable threads about time travel and the modus operandi to each is different.My personal favorite is the one I outlined.