We have a dell server. This server support now for ESXi version: 6.0.0, still we want to upgrade to the new version ESXi Version
You can download the Dell Custom ISO or OfflineBundle for 6.5 and upgrade your existing 6.0 installation. If you boot the ISO it will detect an existing ESXi installation and will ask what to do.
Well, installing a newer ESXi version than officially supported may work, but it's basically a hit or miss.
What you should at least ensure, is that the CPU, the storage adapter (RAID controller), and the network adapters are supported.
Also ensure that you have a current backup of your VMs in case something doesn't work as expected!