SQL Server 2012 on a Windows 2008 R2 cluster, would like to upgrade Windows to 2012 R2
Here are the steps that I followed to get a Windows 2008 R2 cluster hosting SQL Server to Windows 2012 R2. 

In this example we have a 2 node Active / Passive cluster running Windows Server 2008 R2 Enterprise hosting SQL Server 2012 SP2 with CU6, this will work with any version of SQL Server.

Step 1: Document the following:

Drive letter mapping to shared storage on each node, NTFS Shared folders, IP Addresses for LAN and Heartbeat on each node, Name and IP address of the Windows Virtual Cluster Node, Name and IP address of the SQL Virtual Cluster Node, SQL version, patch level, and features installed, The location of the System Databases

Step 2: In Failover Cluster Manager – Evict the passive node, in this example Node 2 is the passive node.

Step 3: Rebuild Node 2 with Windows Server 2012 R2, using the same server name and IP address. Verify that Node 2 can see the shared storage, but do not make them online at this point. 

Step 4: Stop the SQL Services on Node 1.

Step 5: Locate the folder containing the system databases and move them to a secure location. Make sure you get all the system database including TempDB, MSDB, report server, and distribution if you have replication.

Step 6: Shutdown Node 1.

Step 7: In Active Directory Users and Computers disable the accounts for the Windows Cluster Node and SQL Cluster Node.

Step 8: On Node 2, Create the Windows Failover Cluster using the previous Windows Cluster Name and IP. At this point the shared drives will be present, verify they have the correct drive letters.

Step 9: Install SQL Server as a cluster using the same name, IP address, include all features required. 

Step 10: Patch SQL Server to the correct SP and CU.

Step 11: Stop the SQL Services using SQL Server Configuration Manager.

Step 12: Copy the system database files that you saved from Step 5 over top the system database files from this new installation.

Step 13: Start the SQL Services using SQL Server Configuration Manager, verify all databases and logins are available. 

Step 14: Install Windows Server 2012 R2 on Node 1 and join the Windows Failover Cluster.

Step 15: Install SQL Server as a new node to the SQL Cluster, patch to correct level.