To change between single and double sided printing, when you hit print (CMD+P or FIle > Print...) you need to get to Mac's Native System Printing Dialog - meaning not an individual program's print menu which you'll see in apps like the Adobe Suite or Google Chrome. In Chrome, for example, that means clicking 'More Settings', then scrolling the bottom and selecting 'Print Using System Dialog'.

From the System Print Dialog Presets page ("see details", if options are not showing), you should see a dropdown filled with Layout. Click Layout and choose Finishing. On the Finishing tab, you can select between 'Single Sided Printing' and 'Double Sided Printing'.