First, I want to make it clear that you can not natively do a Set Column action on multiple rows. The only thing you can do against multiple rows is a Delete Row action.
‘This Item’ is the current row attached to the screen you were viewing when you clicked on the button.
To add to what @Darren_Murphy shared, that method should work, but since your are dealing with quantities instead of true/false, like that example is doing, you may need some additional logic, after you reset the rows, to reset quantity for a row when you click on, or make a change, in that row. So you would do a Set Column to zero before adding the quantity. This would happen when you set a new Unique ID to mark a row as selected.
I’ll also mention that the Reset Rows method mentioned above is a precursor to this example below, which does deal with quantities, so may be a better example to look at.