If you do not have a disk, visit www.office.com/setup to get started installing and using Office for the first time.
Download, install, or reinstall
Most versions of Office can be managed by logging into www.office.com/myaccount with your Microsoft Account.
Depending on the type of Office you purchased, you can do the following on the My Account page:
Office for a single PC (Office Home & Student 2013, Office Home & Business 2013, or Office Professional 2013)
Download and reinstall Office on the same PC.
Order or burn backup copies of Office.
Source from: http://go.microsoft.com/fwlink/p/?LinkID=403683