declare @testo varchar(1000);
SELECT 
@testo=replace(XML_FIELD.value('(property[@name="items"]/value)[1]', 'nvarchar(1000)'),'PIPPO','PLUTO') 
FROM TABELLA_XML
where cmp_cod='111';
updateTABELLA_XML
set XML_FIELD.modify('replace value of (property[@name="items"]/value)[1] with sql:variable("@testo")')
where cmp_cod='111';