It seems to be working.

SQL

-- DDL and sample data population, start
DECLARE @tbl TABLE (ID INT IDENTITY PRIMARY KEY, Cd_CF VARCHAR(10), NoteXML XML);
INSERT @tbl (Cd_CF, NoteXML) VALUES
('C000001', N'<rows>
   <row nota="2">Contratto depositato [...] :-) </row>
   <row nota="3">
       Gli è stata concessa una dilazione.
       Porre attenzione alle condizioni.
    </row>
    <row nota="49">deadline details</row>
</rows>');
-- DDL and sample data population, end

DECLARE @replaceWith VARCHAR(30) = 'Deadline Balance';

-- before
SELECT * FROM @tbl;

UPDATE @tbl 
SET NoteXML.modify('replace value of (/rows/row[@nota="49"]/text())[1] with sql:variable("@replaceWith")')
WHERE Cd_CF = 'C000001';

-- after
SELECT * FROM @tbl;

Output

<rows>
  <row nota="2">Contratto depositato [...] :-) </row>
  <row nota="3">
       Gli è stata concessa una dilazione.
       Porre attenzione alle condizioni.
    </row>
  <row nota="49">Deadline Balance</row>
</rows>

You can cast the @nota attribute to an integer in your XQuery expression. Here's the modified query:

UPDATE CF
SET NoteXML.modify('replace value of (/rows/row[@nota=49]/text())[1] with "Deadline Balance"')
WHERE Cd_CF = 'C000001'

Or else you can explicitly cast the @nota to a string by using below query

UPDATE CF
SET NoteXML.modify('replace value of (/rows/row[@nota = string("49")]/text())[1] with "Deadline Balance"')
WHERE Cd_CF = 'C000001'