To change the page number location, you must first be logged as the Admin for your book since only the Admin may change the page number location. If you do not see Admin in the top-level toolbar, shown in the screenshot below, then you are not logged in as the Admin.
Once you are logged in as the Admin, click on the Ladder in the top-level toolbar.
Then click on the Page Number button in the second-level toolbar.
Notice how you have many choices for location, color, typestyle and background shape associated with your page numbers. Any choices you make on the Page Number screen will be carried throughout your entire book. Once you have selected the options you want, press Save.