If you'd taken care to read the MSDN page on the ALTER TABLE command (
ALTER TABLE[
^]) you would have noticed that the commands BNF description does not allow for it. All you can do would be to
DROP
the constraint an reinstate it with the correct parameters.
Example of adding a constraint to an existing table:
ALTER TABLE [wm].[TABLE_NAME] WITH NOCHECK ADD CONSTRAINT [FK_TABLE_NAME_PARENT_TABLE_NAME] FOREIGN KEY([FOREIGN_KEY])
REFERENCES [wm].[PARENT_TABLE_NAME] ([PRIVATE_KEY])
ON DELETE CASCADE
Cheers!