You can change fields on Cards in bulk from a search in your Project Library.
Hit the bookcase icon on the far left of your screen to access the Project Library.
Select "Cards" and filter your search by card category, fields, or type a keyword.
When you have all the Cards you wish to change, select onethen hit the double ticks that appear in the bulk actions toolbar to select all.
Choose the "Change Field Values" option and edit the fields of all Cards you have selected at once. Hit "Save" when you are done.
Note: for multiple-choice fields, you can choose to either "Add to existing" fields selected or "Replace all" with the new fields you choose.