CREATE FUNCTION dbo.udf_SubstituteChars
(
@ReplaceList VARCHAR(50),
@String VARCHAR(100),
@NewString VARCHAR(50)
)
RETURNS VARCHAR(100)
AS
BEGIN
DECLARE @CHAR CHAR(1),
@Loop INT

SET @Loop = 0
WHILE @Loop < = LEN(@ReplaceList)
BEGIN
SET @Loop = @Loop + 1
SET @CHAR = SUBSTRING(@ReplaceList, @Loop, 1)
SET @String = REPLACE(@String, @CHAR, @NewString)
END

RETURN @String

END

Then execute

select dbo.udf_SubstituteChars(‘a’,column_name,’#')
from table_name

It will replace all a’s in the column_name with ‘#’s in the table