I don't think you need to change the value of the field at all. Use a field of type number for boolean fields. Using data formatting from the inspector's data tab, you can then specify that values of 1 be displayed as "yes" and values of 0 as "no".
And the user entering data in that field uses...?
a 1 or a 0, but that can be supplied by a value list or scripted buttons.
If you prefer to store the text as yes and no, replace field contents with the calculation option can be used.
Put the field on a layout and put the focus on the field by clicking or tabbing into it. If it's formatted with radio buttons or such, click one radio button then the other to keep the value unchanged, but still put the focus in the field.
Select Show All Records from the Records Menu
Select Replace field contents from the Records menu and select the calcualtion option.
Use an expression such as:
If ( YourBooleanField = 1 ; "yes" ; "no" )
This assumes that you have a single number, 1 in the field and that you want the value "no" to be entered into the field if 1 is not currently stored in the field.
Since this will change the value of this field for every record in the table and cannot be undone, save a back up copy of your file before trying this out.
Ps. if your field if of type Number, change it to text after making this change.
OK I was able to make a script using Replace Field Contents with that condition as the Condition, and no "Specify Target Field: that is what was also confusing me.