update tab
set content=replace(content,'cd','0')
where charindex('cd',content)>0

------解决方案--------------------
SQL code

create table os(a text)
insert into os select 'abcdef'

DECLARE @a binary(16)
SELECT @a = TEXTPTR(a) from os
UPDATETEXT os.a @a 2 2 '0' 
GO

select * from os

------解决方案--------------------
SQL code

update 表 set content=replace(content,'cd','0')
where charindex('cd',content)>0