How do I change the index: add a term, remove one, rename or combine them?

Open the book, click "Index". To add a heading, type it in "Add a heading of your own" and click Add: BookyAI shows at once how many places mention it. Click any heading to open its panel: change the heading as it is printed, add another wording it is found as (a short name, an abbreviation), file it under a broader heading with "File under" (it prints as a subentry, for example "taxes: deductions"), combine two headings with "Merge into" (both wordings are kept), and add related headings under "See also". The panel lists every mention in context; click one to open the editor on that passage. On each row, the eye icon keeps a heading out of the book without deleting it, and the bin deletes it. "See it as printed" shows the index the way the book prints it. "Compile again" reads the whole book again: headings you added yourself are kept, the AI's proposals are replaced.