If you've been using the free version of MS Office Suite, and the trial period is over, then you must go to www.office.com/setup to purchase a brand new product to Office Setup or renew the old Office Setup product. When you purchase your new package, you'll receive the Office setup product key to your email address that you have registered. This key is required to enable the software when creating Office Product.
If you own the Office setup product key, but don't already have Microsoft Office installed in your PC or other devices, you'll need to go to Microsoft Office's website. Microsoft Office website, whether on a desktop or online, it is essential to have a valid product key to activate or reactivate the software.