The world has changed and you need to revise the entire book thoroughly. Perhaps you need to write a new chapter, or you need to make changes to particular text chunks throughout the book. Then you possibly need to produce a revision of your first text.
It is wise to copy the entire Brain to a new location to do this work, so that you original Brain and all the associated files with the previous version can be maintained. If you need to produce a book from the previous version you always have it.
Using the copy of the original Brain you now need to follow the process through from the beginning. Do some brainstorming there in the “Topic” section of the Brain. Then, follow the process through, building new chapter thoughts, and linking thoughts with them and so on. You will be surprised how a different text can come out of the original and how many of those original text chunks can be re-used.
Though it is tempting, it is not wise to simply write a new chapter and tack that onto the old version. First, you do not give your current readers anything really new except that one or two new chapters. You could have simply given your current readers those two new chapters. Second, adding the new information may change the way some of the other chapters are explained and some of the illustrations and other materials may need changing. Third, you have learnt so much more since writing that original volume, make that learning inform the whole text and not just the two new chapters.
So, even though you are starting with a pre-made Brain, go through and do some more brainstorming, review the old brainstorming, delete the ideas that should not be there anymore and add anything new. Follow the entire process through including generating a new cover and a new Foreword –you have moved on since you wrote the last Foreword, it is time your readers saw what made the difference and brought about this revision.