You can edit the field values of multiple cards at once in the Project Library.
Click the bookcase icon
on the far left of your screen to access the Project Library.
Select the "Cards" tab.
Filter your search by card category, fields, or enter a keyword.
Once you have all the Cards you wish to change, tick the checkbox to select one Card.
Then click the double ticks 
at the top of the bulk actions menu to select all.
Click the "Change Field Values" icon
in the bulk actions menu.
The "Update Card Fields" dialog will list all the fields that are common to the selected Cards. You can add, change or delete a field values.
Select the changes you want to make, and click "Save".
Then click "Confirm".
Note: for multiple-choice fields, you can choose to either "Add to existing" fields, or "Replace all with" the selected fields.
For more help on fields see the Fields Help Guide.