First, go to the Yearbooks tab on the Management page. After you find the project that you want to update, click on the dropdown icon to the right of the green Yearbook or yellow Subjects button. One of the options will be "Copy From Previous Year". This will only show up and work if you have another yearbook project in the same school project to clone from.

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After clicking "Copy From Previous Year" a dialog with your choices should appear. Select the project you want to copy from, then click OK.
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After it finishes copying, there will be a popup to indicate it is finished copying the book. When you go into the Book now you will see all of the pages laid out similar to how they were in the other yearbook project. Backgrounds and placeholders are copied, but actual candids and subjects are not. The class layouts will be shown as placeholders until you drag the new batches with this year's subjects onto them.