You will need the ISO image downloaded from VMware's website, but you would also need a method to present the ISO to the physical machine so that it can boot from it. Most modern server equipment has a methodology to avoid the need to burn ISO to a physical DVD medium and then insert it into the DVD drive of the physical machine. If you are an administrator, you might already be aware of terms such as ILO (HP), DRAC (Dell), and KVM Manager (Cisco).
These are web-based tools that will connect to a Remote Access Card (RAC) on the server and enable remote access to the server's console through the web. Enough said on what is available out there; let's make a list of what you need to begin the upgrade:
- The ESXi 6.5 hypervisor ISO image downloaded from VMware's downloads page https://my.vmware.com/web/vmware/downloads
- Access to the remote console of the server on which the installation will be performed