How do you update the default value for a field programmatically? How do you update the default value for a field programmatically?  I know you can get the field instance info by calling field_info_instance, but how do you update that info?
The field_update_instance() function is the standard method


  Updates an instance of a field.


You can take the array returned by field_info_instance(), make your changes, then pass it back to field_update_instance() and the changes will be saved.