To tell a windows server to use a specific Windows Server update Services (WSUS) server, use the following:

 Start > Run > gpedit.msc > OK > Computer Configuration > Administrative Templates > Windows Components > Windows Update

 The 2 keys 'Configure Automatic Updates' and 'Specify intranet Microsoft update service location' are the items you will want to change.

Periodically, I have a base image that needs to be updated.  It is usually not in a running state so it falls behind in Microsoft patches.  After waking the VM from storage, I use the following to force a check to the defined Windows Server Update Services (WSUS) with the following command from the command line:

wuauclt /detectnow