As with all other collection management, you go to the collection you wish to edit and on the left hand pane, select SEARCHES.
Here you will see your existing set-up searches.
You have the option to EDIT a current search or CREATE a new instance of a search.
Once in the search window, you simply drag across the fields to create conditions for the search (or add a section) to enforce criteria. You then choose an appropriate operator for your new field and save any changes.