Implement GridView_RowDataBound event and into this check the value of dropdown. And after that change the item accordingly.
Other option is to user CASE clause in your query and get the exact data from database as you want to display.
Modify queyr as below
DropDownColumnName = CASE tablecoulmname
WHEN tablecoulmname = 1 THEN "Yes"
WHEN tablecoulmname = 0 THEN "No"
ELSE defaultvalue as per your requirement
Hope this will help you.