--Altering an existing column to add a default constraint:
ALTER TABLE {TABLE NAME}
ADD CONSTRAINT {CONSTRAINT NAME}
DEFAULT {DEFAULT VALUE} FOR {EXISTING COLUMN NAME}

--Adding a new column, with default value, to an existing table:
ALTER TABLE {TABLE NAME}
ADD {COLUMN NAME} {DATA TYPE} {NULL | NOT NULL}
CONSTRAINT {CONSTRAINT NAME} DEFAULT {DEFAULT VALUE}

--Dropping a contraint:
ALTER TABLE {TABLE NAME}
DROP CONSTRAINT {CONSTRAINT NAME}