Please see following documentation to install and setting up UMDS (Update Manager Download Service) for 6.5
Install UMDS on Linux:
https://techdocs.broadcom.com/us/en/vmware-cis/vsphere/vsphere/6-5/vsphere-update-manager-installation-and-administration-6-5/installing-setting-up-and-using-update-manager-download-service/installing-umds-on-linux-os/install-umds-on-linux-os.html
Install UMDS on Windows:
Setting up and using UMDS:
Note: vmware-updateDownloadCli.exe is located in the default Update Manager install folder (C:\ProgramFiles\VMware\Infrastructure\Update Manager). This command is not available for VMware Update Manager 4.0 and later. For more information, see Use a Shared Repository as a Patch Download Source in the VMware vCenter Update Manager Administration Guide.
Note: This command is only to download the updates for an ESX host. For downloading Windows and Linux updates, see the VMware Update Manager Administration Guide .