If you’ve updated or re-installed your Windows and need to reinstall MS Office 2016 program again, you should find out Office 2016 or 2013 product key first then continue the installation.īut what to do if you have lost or forgotten 25-digit code key? Your first thought might be that Windows Registry. When you purchases MS Office from the Microsoft store, it comes with activation or product key which can be used to activate the application during the installation process, without it, you will neither get the free updates nor official support, even restrict certain functions.