Add DEFAULT to existing column In Microsoft SQL server, how can I add/change the default on an existing table column using T-SQL?
ALTER TABLE yourTable ADD  CONSTRAINT constraintName  
   DEFAULT ('XYZ') FOR [YourColumn]


To change the default, drop the constraint and re-add it with the new value:

ALTER TABLE yourTable
DROP CONSTRAINT constraintName

ALTER TABLE yourTable ADD  CONSTRAINT constraintName  
   DEFAULT ('ABC') FOR [YourColumn]