If you :
Depending on how you purchased Office 365, your product key might be located in different places: office 360 product key
To activate new purchases, renew subscriptions, or install licensed software on a new PC 1.2.1 , 1.2.5. How to Find Your Office 365 Product Key If you : Depending on how you purchased
If you already have Office installed but lost the key, you can try: you can try: