If you are already in the Composer, first go back to your Dashboard by clicking on the button in the top right of your screen, and then clicking Dashboard.
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Once you are on the Dashboard, click on the Yearbooks tab on the left. Find the yearbook that you want to edit, then click on the down arrow to the right of the green Yearbook button. Click on Project Settings from the dropdown that appears.

When you are in the Project Settings, you can edit the number of pages your book should have. After you change the number, click Save and the new pages should be in your Book to use!
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