You can apply this setting in two parts of the process:
1. In the editor.
1.1 Click Preview, on the upper-right corner of the screen.
1.2 - Select one of the double-sided options
2. In the print screen (after you've entered your billing information).
2.1 - Click on Change page layout
2.2 - Select one of the double-sided options and save.
2.3 - Regenerate the PDF file to apply the changes. You need to click the regenerate icon:
2.4 - Click the new document to download and print.