You edit the search view by navigating to the list for the search you wish to modify. In this case, we will simply click on Sales Orders under the main Sales menu. Then, under the Debug View menu, choose Edit SearchView.
The XML for the search view will appear for you to edit, as shown in the following screenshot:
Notice that there is an inherited view specified. This means that the view you are looking at is inherited from sale.order.list.select. Click the small icon to the right of the inherited view field to pull up the view it is inherited from.