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.

From that screen, look for the Seasons dropdown. Change it to be the season you want it to be in, and then click Save at the bottom. Now the Book project will be in a different season!