Log in to me&u's Operator Portal. Click on "Launch Venue Manager". You will be redirected to a new window - Venue Manager, where you will have enter your pin.
Click on "Availability", filter for either Products or Modifier and search for the item you would like to mark as sold out.
Click the "Available" icon on the item, and "Unavailable" and "Sold out today" will appear. "Unavailable" means that the item will remain permanently sold out until it is made available again where as an item marked "Sold Out Today" will automatically return to “Available” the next day.
You can also click on "Show unavailable" to filter for all products or modifiers that are currently unavailable.