I would add that you can also do it like this:

--sample data

SELECT * INTO #test

FROM (VALUES (1,'blah AAA'),(2,'bbb bbb bbb'),(3,'AAA ccc')) t(id,val);

SELECT * FROM #test;

WITH your_table AS

(

SELECT * FROM #test WHERE val like '%AAA%'

)

UPDATE your_table

SET val=REPLACE(val,'AAA','[xxx]')

SELECT *

FROM #test

I like this technique because, in SQL Server Management Studio, you can highlight & execute just the SELECT * FROM #test WHERE val like '%AAA%' portion of the statement to see what rows are going to be updated.