First for the workflow, it is set to be triggered when the library is changed,here the library i am going to use is called doc library.
Then according to your description, i will add an update list item action to update the dummy list dashboard. You will need to add the filed you want to update in the list and set its value to required value. Just like the pic below.
You can see in the "To this value" field, i set it to be the current item:Status. It means, when the doc library has a file been modified, the workflow will try to update the list item field "status" in the dashboard list. The only problem remain is which item shall the workflow to update?
In fact you can find the introduction comment "Choose a field from the selected list and matching value that identifies the specific item you want from the data source"
In my example, the setting means, when the workflow triggered, it will find the list item in dashboard which has the same title with the current item( the current item means the file/item which is modified and trigger the workflow)
Note: Hope this will not confuse you, the settings in "Find the list item" simply help us to get the required item. We are kind of setting up an equation, on the left is the list item field, and on the right, is the field we set in the "value". When the equation is met, the workflow will do the update we set in the upper part.
