Upgrading from 5 to 6.5 can take quite a while! So get a snack!
You'll need to login into your ESXi host via SSH. This isn't enabled by default so you'll have to enable it first.
Once logged in, here are the commands used for the upgrade:
vim-cmd hostsvc/maintenance_mode_enter
esxcli system version get
esxcli network firewall ruleset set -e true -r httpClient
esxcli software sources profile list -d
https://hostupdate.vmware.com/software/VUM/PRODUCTION/main/vmw-depot-index.xml | grep -i ESXi-6
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
vim-cmd hostsvc/maintenance_mode_exit
reboot