Open heart surgery
Basically, patient is put on heart
-lung machine (cardiopulmonary bypass), so right atrium can be opened, and a patch (usually from pericardium) is sewn in to close the atrial septal defect (or hole in the wall between right & left atria of heart) .
.. But can be done endovascularly by cardiologist with device that goes across ASD and opens up to cover ASD on both sides of asd.