Unfortunately, I don't think you can do it via a macro, but there may be am alternative.
First, lets make sure we are on the same page here. I'm guessing what is happening is that you are doing data entry on a bunch of items where some of the data keeps repeating. For example, you might be entering a bunch of addresses from the same city and state so you want to default to that city and state. But you want to change the City and State when you start on the next batch. If that's the case then a macro won't do it.
The concept here is that you want to change the default value for the NEXT record to equal the data value in the current record. To make it clear, the Default Value for a control only takes effect when you start a new record. So you enter an address for Boston, MA, then you go to the next record and Boston MA is filled in automatically.
If that's the case, then a macro won't work because there is no way to set the Default property in a macro. There is a Set Property macro action, but Default is not on the list of properties that can be set.
But, as I said, there is a work around. What you can do is put some unbound text boxes on your form (usually in the form header). Then you can use the ON Current event of the form to run a macro that assigns the value in those unbound text boxes to the bound controls on the form.