Hi Skip,
The best thing to do is add the chapter to the book you have already started. There is no need to make the first chapter inactive. You might have new reviewers who come along and want to start from the beginning.
To do this, click the blue Post Your Writing button at the top of the page. Find and click on the title of your book. Go to the Content sub-tab and click the big blue Add Chapter button. Then, follow the steps to publish the chapter.
If you still want to make the previous chapter inactive, you can do that by following the same steps but going to the Publish sub-tab. From there, find the chapter and switch it to inactive.
Let me know if you have any other questions. There is also a video you can watch on publishing process which you can find here:
http://www.thenextbigwriter.com/member/video-help-guide
Best,
Sol