How to move GI Bill months between dependents?

Ex and I both veterans but I got out before moving/leaving to kids GI Bill became a thing so I’m unfamiliar with the process.

Ex left 2 years to each of our kids. One kid used a year and is done. Won’t use the second year.

Ex used most/all of second kid’s months and went back to school himself.

The second kid is now entering college in the Fall and needs the remaining year from kid 1 moved to her. I’m trying to help her apply for benefits.

What does she or her dad need to do to move the remaining months to her?

He’s not always on top of stuff so I’m trying to understand the process so I can encourage/facilitate this getting done.

Thanks for the help!