Download the Server Extension installer from this page.
Note: You must install the Server Extension on your SharePoint web front-end servers (including where your Central Administration is hosted, which is also a web front-end server). If you have more than one Web Front-End server, copy the installer into all of them, then follow the install procedure. Remember, your SharePoint server must have at least .NET framework 3.5 or higher in order to proceed with a Server Extension installation.
Installing the Server Extension will not cause any downtime on your SharePoint server and does not require a server reboot.
Launch the Extension installer, and follow the path as specified:
Note: The Sandbox Server Extension has been deprecated, and will not be required when migrating to Office 365.