Updating multiple attribute in XML using "replace value of" is challenge as, we are constraint to update 1 value at once. To achieve this we need to loop for each attribute.

Below is script used in lesson:

DECLARE @T TABLE
  (
     xmltext XML
  )

INSERT INTO @T
VALUES     ('<Configuration>   <OptIn>     <CodeId IncludeChildren="True">2548578015</CodeId>     <ReadCode>93C0%</ReadCode>   </OptIn>   <OptOut>     <CodeId IncludeChildren="True">2533649013</CodeId>     <CodeId IncludeChildren="True">2548920016</CodeId>   </OptOut> </Configuration>'),
            ('<Configuration>   <OptIn>     <CodeId IncludeChildren="True">2548578015</CodeId>     <ReadCode>93C0%</ReadCode>   </OptIn>   <OptOut>     <CodeId IncludeChildren="True">2533649013</CodeId>     <CodeId IncludeChildren="True">2548920016</CodeId> <CodeId IncludeChildren="True">2548920016</CodeId>   </OptOut> </Configuration>'),
            ('<Configuration>   <OptIn>     <CodeId IncludeChildren="True">2548578015</CodeId>     <ReadCode>93C0%</ReadCode>   </OptIn>   <OptOut>     <CodeId IncludeChildren="True">2533649013</CodeId>     <CodeId IncludeChildren="True">2548920016</CodeId>   </OptOut> </Configuration>')

SELECT *
FROM   @T

WHILE EXISTS (SELECT 1
              FROM   @t
              WHERE
xmltext.exist('/Configuration/OptOut/CodeId[ contains(@IncludeChildren , "True")]') = 1)
  BEGIN
      UPDATE m
      SET
xmltext.modify('replace value of (/Configuration/OptOut/CodeId[ contains(@IncludeChildren , "True")]/@IncludeChildren)[1] with "false" ')
FROM   (SELECT xmltext,
Replace(t.u.value('./@IncludeChildren[1]', 'varchar(100)'), 'True', 'False') AS
replaceval
 FROM   @t p
        CROSS apply
p.xmltext.nodes('/Configuration/OptOut/CodeId[ contains(@IncludeChildren , "True")]')t(u))m
END

SELECT *
FROM   @T