It is all explained in the lectures: Chapter 38 for B&HH and Lewin, and Chapter 36 for POPIT.
POPIT relates to the areas that might need to be changed and which are all interrelated.
B&HH and Lewin are how to manage the changes so that, for example, staff are willing to accept them or the change process is otherwise successful. Which you use depends on the information in the question.