VMware ESXi 5.5 allows you to upgrade from previous ESXi versions using several different methods. These methods are described and outlined in VMware Knowledge Base article Methods for upgrading to ESXi 5.5 (2058352).
In this new video tutorial we demonstrate how you can upgrade from VMware ESXi 5.1 to ESXi 5.5 using the Interactive Installer that is supplied on the ESXi 5.5 Installation Media. We’ll show you the step-by-step procedure for performing a basic upgrade from ESXi version 5.1 to version 5.5 using the Interactive Installation method, however the instructions apply to older versions of ESXi as well.
For information relating to the other upgrade methods, see VMware Knowledge Base article Methods for upgrading to ESXi 5.5 (2058352).