See example from help documentation. You can use SELECT INTO clause.
CREATE PROCEDURE upsert_proc (IN v_isbn VARCHAR(20))
LANGUAGE SQLSCRIPT AS
BEGIN
DECLARE found INT := 1;
SELECT count(*) INTO found FROM books WHERE isbn = :v_isbn;
IF :found = 0
THEN
INSERT INTO books
VALUES (:v_isbn, 'In-Memory Data Management', 1, 1,
'2011', 42.75, 'EUR');
ELSE
UPDATE books SET price = 42.75 WHERE isbn =:v_isbn;
END IF;
END;