You need to create an update query based on your table, bring down the field you want to update (Column 2) and the column you are using as your criteria (Column 1). In the 'Update To' section beneath Column 2 put in 1, leave the 'Update To' section beneath Column 1 blank. In the criteria section beneath Column 1 put "Yes" and leave the criteria section beneath Column 2 blank.
As with any action query, make a backup of the table you are working with just in case things don't go as planned.