How do I change one page?

Show it (click it in the Designs column), then in the page toolbar: choose another kind of design, another shape (silhouettes and mystery mosaics), other cells (mystery mosaics), click the round arrow for "A new design of this kind", "Edit" to change its lines, "Change colors", the bin to remove it (the app asks "Remove design N from the book?"), or the lock to keep it when the book is drawn again.