What do you mean by "taking over"? Are you the new Product Owner? A new coder? A one-man show doing everything? Is this part of a larger organization, or did you just pick up a codebase and you are going to try to make it into a business?
I'm the new product owner, I don't code but manage the requirements for the team that does. It's an existing product which has been in life for many years and is part of a large organisation.
Gotcha. In that case, I agree with some of the other comments. Sitting down and talking to the users to understand the pain points and ongoing needs is most important. Past history is not as critical because it is your job to define the future, not maintain the past.