How to UPDATE just one record in DB2?
Actually, on the link you have, it mentions that you can update only one record, you just have to use cursors to do so. It then points you to the DECLARE CURSOR page.

So first, you would have to declare a cursor from your table with the select statement that would give you what you want.

DECLARE C1 CURSOR
FOR (SELECT * FROM MYTABLE WHERE...);


Move to the value of the record in the cursor that you want.

Then run you update statement against the cursor using the link you mention

UPDATE MYTABLE 
SET MYCOLUMN = ?
WHERE CURRENT OF C1;


I believe you need to issue a COMMIT (which closes the cursor unless you specify otherwise in the DECLARE CURSOR clause).

Check out the documentation in the link you have and in the one I have. You may end up having to use a stored procedure to run this.