Working in my lab and figured I’d outline the steps I used to update my ESXi 6 host to 6.5:

Enable SSH on your host

Run this CMD to display the list of available updates: esxcli software sources profile list -d https://hostupdate.vmware.com/software/VUM/PRODUCTION/main/vmw-depot-index.xml | grep -i ESXi-6.5



Then this CMD to install the 6.5 update: esxcli software profile update -p ESXi-6.5.0-4564106-no-tools -d http://hostupdate.vmware.com/software/VUM/PRODUCTION/main/vmw-depot-index.xml

Once it completes you’ll be prompted to reboot your ESXi Host for the update process to complete.