This might hep for n number of columns :

*********************************************************************************************

DECLARE @Q NVARCHAR(4000),

@CName VARCHAR(255)

DECLARE ColCur CURSOR

FOR

SELECT C.name as Column_name

FROMsys.columns C

LEFT JOIN sys.tables t

on C.object_id = t.object_id

WHEREt.name = 'Your_table'

OPEN ColCur

FETCH NEXT FROM ColCur INTO @CName

WHILE(@@FETCH_STATUS =0)

BEGIN

SET @Q = 'UPDATE Your_Table SET '+@CName+' = NULL WHERE '+@CName+' ISNULL '

PRINT @Q

FETCH NEXT FROM ColCur INTO @CName

END

CLOSE ColCur

DEALLOCATE ColCur