You won't be able to set the value of an individual field in a fieldgroup from CBSubs. It's a lot more complicated to change the values of field group fields as the value of such a field is actually just a JSON string. You'd need some like a Code action in CB Auto Actions to convert the JSON to a PHP array, parse through the rows of the array, find the row you want to change, and then change the field value in that row. I don't recommend using a field group field if you need to do value switching like that.