How can I change the homepage in our handbook?

To do this, you must be an administrator in the handbook. Go to the handbook where you want to change the homepage image, and then enter edit mode. 

You can do this by clicking on the round button with three dots and selecting "Edit". Next, click on "Administration" in the blue bar at the top, and then select "Homepages" from the menu on the left side. 

Here, you will see the existing homepage and any others that have been created. You can either click on the homepage name to edit it or click "Create" in the top right corner of the screen to create a completely new one.