The default printer is the one selected when the Print dialog opens.

If you see “No printer selected” in the Print dialog, add a new printer. The first printer you add becomes the default printer, but you can change the default printer at any time.

Change your default printer

  1. On your Mac, choose Apple menu  > System Settings, then click Printers & Scanners in the sidebar. (You may need to scroll down.)The Printers & Scanners dialog showing the Add Printer, Scanner, or Fax button.
  2. Click the “Default printer” pop-up menu on the right, then choose an option.
    • If you want the same printer to always be the default printer, choose that printer. The default printer doesn’t change even after you print to a different printer.
    • If you want the default printer to be the one you printed to last, choose Last Printer Used. Your Mac remembers the printer you used last at your current network location. For example, the printer you used last might change depending on whether you’re at home or at work.

Tip: To quickly select a default printer, Control-click a printer in the Printers list of the Printers & Scanners settings, then choose “Set Default Printer” from the shortcut menu.

More information can be found here