MERGE doesn’t need “multiple tables”, but it does need a query as the source. Something like this should work:
MERGE INTO mytable d
USING (SELECT 1 id, 'x' name from dual) s
ON (d.id = s.id)
WHEN MATCHED THEN UPDATE SET d.name = s.name
WHEN NOT MATCHED THEN INSERT (id, name) VALUES (s.id, s.name);
Alternatively you can do this in PL/SQL:
BEGIN
INSERT INTO mytable (id, name) VALUES (1, 'x');
EXCEPTION
WHEN DUP_VAL_ON_INDEX THEN
UPDATE mytable
SET name="x"
WHERE id = 1;
END;