Sometimes XML questions get no love around here.

I am not great with the XML modify method (something I need to read up on and practice I guess) but here's an approach: we can cast the XML as a string and STUFF your row where required as shown here:

-- sample table containing your xml

DECLARE @table TABLE(id int identity, xcol xml);

INSERT @table(xcol) VALUES(

'<row id="1000000" xml:space="preserve">

<c1>Exported</c1>

<c2>Text Sample 1</c2>

<c2 m="2">Text Sample 2</c2>

<c2 m="3">Text Sample 3</c2>

<c2 m="4">Text Sample 4</c2>

<c3>Text Sample 1</c3>

<c3 m="2">Text Sample 2</c3>

<c3 m="3">Text Sample 3</c3>

<c4>Text Sample</c4>

<c5>Text Sample</c5>

</row>');

--Solution

SELECT id,

xcol =

CAST(

REPLACE(REPLACE(STUFF(

CAST(xcol as varchar(8000)),

CHARINDEX('<c4>',CAST(xcol as varchar(8000)))-1,0,'<ctest>New Added</ctest>'),'&#x0A',''),'>;<','><')

AS xml)

FROM @table;

Another approach would be to use mdq.xmltransform if you are familiar with XSLT.