Updating XML modify method

I am trying to update XML string, but I am getting this error:

Incorrect use of the XML data type method 'modify'. A non-mutator method is expected in this context

Code:

DECLARE @xmlVal XML  ='<r k="ABC"><p k="Id">721</p><p k="ConnectionId">17150E3C-2D3B-4760-B392-0DF7563198D1</p><l k="param" /><l k="filter"><l k="Default"><p k="Id">123</p><l k="Exclude"><l k="Exe"><p k="__Id">efbef694-e86f-487b-9360-9596224c9908</p><l k="Criteria" /></l></l><p k="Family"></p></l></l><l k="Methods" /><l k="Groups" /><p k="Default"></p></r>'

SELECT 
    CAST(@xmlVal.modify('replace r/p/p/text())[1] WITH 2BDFF1B4-33CD-4CF9-9AD6-17F28C2F03E0') AS varchar(max))

Please let me know what I am doing wrong to replace text inside p tab with k=ConnectionId

Solution

It is impossible to modify XML via XQuery .Modify() method inside a SELECT statement. It should be two distinct statements.

SQL

DECLARE @xmlVal XML =
N'<r k="ABC">
    <p k="Id">721</p>
    <p k="ConnectionId">17150E3C-2D3B-4760-B392-0DF7563198D1</p>
    <l k="param"/>
    <l k="filter">
        <l k="Default">
            <p k="Id">123</p>
            <l k="Exclude">
                <l k="Exe">
                    <p k="__Id">efbef694-e86f-487b-9360-9596224c9908</p>
                    <l k="Criteria"/>
                </l>
            </l>
            <p k="Family"></p>
        </l>
    </l>
    <l k="Methods"/>
    <l k="Groups"/>
    <p k="Default"></p>
</r>';

DECLARE @d UNIQUEIDENTIFIER = '2BDFF1B4-33CD-4CF9-9AD6-17F28C2F03E0';

SET @xmlVal.modify('replace value of 
      (r/p[@k="ConnectionId"]/text())[1]
      with sql:variable("@d")');

-- test
SELECT @xmlVal;

Output

<r k="ABC">
  <p k="Id">721</p>
  <p k="ConnectionId">2BDFF1B4-33CD-4CF9-9AD6-17F28C2F03E0</p>
  <l k="param" />
  <l k="filter">
    <l k="Default">
      <p k="Id">123</p>
      <l k="Exclude">
        <l k="Exe">
          <p k="__Id">efbef694-e86f-487b-9360-9596224c9908</p>
          <l k="Criteria" />
        </l>
      </l>
      <p k="Family" />
    </l>
  </l>
  <l k="Methods" />
  <l k="Groups" />
  <p k="Default" />
</r>