How to upgrade a server operating system that has SharePoint Server 2013 or 2010 installed

This article describes how to perform a full-version upgrade of the operating system of a SharePoint Server 2013 or 2010 server. For example, you can use this procedure when you intend to upgrade from Windows Server 2008 to Windows Server 2008 R2 or Windows Server 2012. If you apply updates that include only cumulative updates or service packs on a SharePoint server, you do not have to use this procedure.

To perform a full-version operating system upgrade on a SharePoint Server 2013 or SharePoint Server 2010 server, follow these steps:

1. Make sure that the intended operating system is supported by the version of SharePoint that you are running. 
2. Unjoin the target SharePoint server from the farm.
3. Uninstall SharePoint through Control Panel.
4. Upgrade the operating system.
5. Rerun the SharePoint Prerequisite Installer.
6. Reinstall SharePoint.
7. Rejoin the upgraded SharePoint server back to the farm.

Other recommendations for upgrading servers:

1. Add new Windows 2012-hosted SharePoint servers to the farm and move the SharePoint services and roles to the new server before removing the Windows 2008-hosted SharePoint servers.
2. If you are planning to use the same server with an OS upgrade, you must consider the availability of roles and services in the farm when performing the activity. We highly recommend that you upgrade one server at a time.