USE tempdb;

CREATE TABLE dbo.foo(
fooID int NOT NULL PRIMARY KEY,
Data xml NOT NULL
);

INSERT dbo.foo VALUES
(1, '<property name="items">
  <name>items</name>
  <type>itemseditor</type>
  <label>Items</label>
  <value>PIPPO</value>
  <description>Edit Items</description>
</property>'),
(2, '<property name="items">
  <name>items</name>
  <type>itemseditor</type>
  <label>Items</label>
  <value>PIPPO</value>
  <description>Edit Items</description>
</property>');

DECLARE @myText varchar(10) = 'PLUTO';

/* Aggiorno l'elemento <value> solo della prima riga */
UPDATE dbo.foo
SET Data.modify('
  replace value of (property/value/text())[1]
  with sql:variable("@myText")
')
WHERE fooID = 1;

SELECT *
FROM dbo.foo;

/* Output:

fooID       Data
----------- ---------------------------------------------------------------------------------------------------------------------------------------------------------
1           <property name="items"><name>items</name><type>itemseditor</type><label>Items</label><value>PLUTO</value><description>Edit Items</description></property>
2           <property name="items"><name>items</name><type>itemseditor</type><label>Items</label><value>PIPPO</value><description>Edit Items</description></property>

(2 row(s) affected)

*/

DROP TABLE dbo.foo;