If you created your Calendar without a book (it can be created here ), you can always edit and change the cover or adjust the content of your Calendar just click on button on the top left corner of the screen, once you are in the editing page, there will be a photo icon on the right side of your calendar. You can add up to 4 pictures per month.
To change the cover, go to your cover page and click on the photos icon on the side of the cover photo and you can select a new one.
If you would like to replace photos of other months in your calendar, you can also click on the same photos icon on the side of the page to add more pictures or replace the ones you don't want.
Once you click on the photos icon, a tab will open at the bottom page where you can select the photos for the page. You can click on the photo to add it to the page or to replace it.
When the changes have been made, click on button to save it.
Keep in mind that when ordering your PastBook Calendar with a book, currently it is not possible to make any changes, however, if you change the cover of your PastBook, also the cover of your Calendar will be updated.