So you have completed performing a rolling upgrade for all of your Hyper-V nodes. And you are wondering what you need to do next. Or When you are performing a health check and you notice a Warning on Cluster Events which says that "Hyper-V Node" established communication with Node "Hyper-V Node 2" and detected that it is running a different, but compatible, version of the operating system...

Here is what to do next

1) Launch Powershell
2) Enter the following Command
Update-ClusterFunctionalLevel
3) Press Y and Hit Enter

And you are done!