Yes, you are right. The UM can be modified directly from the box by typing the whole value. You can use the keywords (auto, none, unset or inherit) as well if you prefer using the keyboard instead of the mouse
i like being able to do this without the mouse, fyi. the less i have to touch the mouse/trackpad, the better. moving from one to the other disrupts flow and rhythm. being able to just type the um with the value is great.