After you know about scripts, go & name the units you want unmanned. After that, create a script using the action in the location, Unit/Status/Make unmanned. In that action, choose the unit named. Just leave the condition as True.
If you want a bunch of units unmanned, there's a simpler way then just copying the "unmanned" action for each unit. Go to the Player List, (Edit > Edit Player List) & create a new player there using the Civilian faction. (You can name it if you want.) Now go create a script using the action Team/Status/Make unmanned instead. For the Team, choose the team name of the player you created. Now select all the units that you want unmanned & in the Object Properties window, & in the Team text box, select the team of the player you created. Now those units will be unmanned.