do you know how to make a query?
make a query (Query/New) pick your table with the field you need to be YES (is the field type set to yes/no in your table?) and have you set the default value to Yes so all new records will be created that way? included.
now, while still in the Query Design view, go to text menu at top pick Query then Update Query. You will see the fields change, now you have an 'update to' field in the columns. type type the word 'yes' there for your field, then hit Run. delete the update query after you are satisfied to eliminate possibility of accidentally updating the fields later when you have some 'nos' in there...