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]

StackExchange Database Administrators Q#3843, answer score: 17