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 Clone Previous. This will only show up and work if you had another yearbook project in the same school project to clone from.
After clicking on Clone Previous a dialog with your choices should show up. Select the project you want to clone from, then click Ok.
After it finishes cloning you will get a popup that it is finished. When you go into the Book now you will see all of the pages laid out similar how to they were in the other yearbook project. Backgrounds and placeholders are copied over, but actual candids and subjects are not. The class layouts will be there as placeholders until you drag the new batches with this year's subjects onto them.