When upgrading to ESXi 5.1, you need to consider several things: Why are you upgrading?, What are you upgrading?, What should you do to get ready?, What is the correct upgrade process?, and finally What should you do after the upgrade is complete?
If you are upgrading to ESXi 5.1, there are some important facts that you should consider first. Upgrading involves many stages and processes that must be performed in a specific order. Many of these processes are one-way and do not provide a "back button." If you do not use care and consideration in your upgrade plan, you could possibly lose important data and configuration; and potentially even lose contact with your servers.