UPDATE your_table SET your_column = NULL;

If you want to "delete" the values in a column in all rows then you need to UPDATE the table; DELETEdeletes rows not values in a specific column. So, in pseudo-SQL this would be:

UPDATE YourSchema.YourTable
SET YourColumn = <New Value>;

What your new value is is up to you; it might be NULL , a zero length string ( '' ), 0 , etc.

Such an operation will be an offline operation though, so for a large table this could result in a lengthy lock. If so, and you're on Enterprise/Developer, you might want to consider DROP ing and re ADD ing the column, provided that nothing else is dependent on said column. This is because adding a column (with a default value) is an online process in Enterprise, as it's a metadata change:

ALTER TABLE YourSchema.YourTable DROP COLUMN YourColumn;
GO
ALTER TABLE YourSchema.YourTable ADD YourColumn varchar(10) NULL CONSTRAINT DF_YourTable_YourColumn DEFAULT '' WITH VALUES;

If you want NULL / NOT NULL a DEFAULTCONSTRAINT , WITH VALUES , etc, is dependant on your needs.

For transparency, I VTC'd after posting this answer, as from the comments on the question, it appears that the question asked and the question the OP wants to ask are not the same. This answer addresses the question asked; it is unclear what the OP is really asking.