create table T(c1 varchar(100))
insert T select '<Aes><Cmd>MAReport</Cmd><Preview>false</Preview</Aes>'
update T set
c1=stuff(c1, charindex('<Cmd>', c1), charindex('</Cmd>', c1)-charindex('<Cmd>', c1)+6, '<Cmd>自己定义</Cmd>')
select * from T
--result
c1
----------------------------------------------------------------------------------------------------
<Aes><Cmd>自己定义</Cmd><Preview>false</Preview</Aes>