Project Upgrade, Options Dialog Box
Use the Project Upgrade page to specify whether Visual Studio automatically changes VSTO Add-in projects to target the version of Office that is installed on the development computer.
To access this page, on the Tools menu, click Options. Expand the Office Tools folder, and then click Project Upgrade.
Always upgrade to installed version of Office
This option is disabled for Visual Studio 2013 because Office projects in Visual Studio 2013 can target only Office 2010 or Microsoft Office 2013 applications. For more information, see Upgrading and Migrating Office Solutions.