Quick'n simple example

😎

USE tempdb;

GO

SET NOCOUNT ON;

DECLARE @TXML XML = '<Attribute>

<A1>6529</A1>

<A2>6529</A2>

</Attribute>';

DECLARE @NNODE VARCHAR(100) = 'A3';

DECLARE @nval VARCHAR(100) = 'New value';

DECLARE @Nt VARCHAR(200) = '<{{@NNODE}}>{{@NVAL}}</{{@NNODE}}>';

DECLARE @NXML XML;

SELECT @NXML = CONVERT(XML,REPLACE(REPLACE(@NT,'{{@NNODE}}',@NNODE),'{{@NVAL}}',@NVAL),0);

SET @TXML.modify('insert sql:variable("@NXML") as last into (/Attribute)[1]');

SELECT @TXML ;

Results

<Attribute>

<A1>6529</A1>

<A2>6529</A2>

<A3>New value</A3>

</Attribute>