Best practise to upgrade Windows OS

Hi All,

I'm looking at upgrading vSphere Essentials Plus 4.1 to 5.5 in the near future. The vCenter Server, and DB, is currently hosted on Windows 2003 Server OS on a physical server. I'll need to upgrade this server to Win2008 R2 before I can upgrade vCenter to 5.5. What is the preferred process for performing this upgrade task using the same physical server?

Thanks,

Andrew