Depends on how you have prepared the book?
If you've used the Book Panel it should be quite easy.
You can use the Chapter Numbering and apply it to every chapter and it will auto update.
Are your working with Paragraph Styles? Adjust them, and create another one for the chapter without the number.
I'm using the Book feature, but the main body, the chapters, are not divided into separate documents. So I can't assign chapter numbers In the Book panel. And I don't want to separate each chapter into a section to assign section numbers. That's more trouble than manually changing the chapter numbers.
Where do the numbers for your chapters appear?
If they are their own paragraph style, make it a numbered list.