A family of Microsoft word processing software products for creating web, email, and print documents.
I'm looking at Win 8 and not 8.1, so things might be a little different.
From the Search charm, search for printer. In the results, you should find and tap/click Devices and Printers. Right-click the desired printer and choose "Set as default printer". That will put a checkmark-in-green-circle on the printer's icon. Each time you start Word, it should use that default printer unless/until you choose a different one in the Print page.