Update one or multiple columns of a table to null is really easy in SQL Server.
UPDATE [Table-Name]
SET
[Column-Name-1] = NULL,
[Column-Name-2] = NULL,
...
WHERE [Conditions..]
SET
[Column-Name-1] = NULL,
[Column-Name-2] = NULL,
...
WHERE [Conditions..]
Of course the WHERE condition is optional.