How can add the cumulative sum from bottom to top?

Create a tmp table and add the num column from top till bottom::

WITH data (num) AS (
  VALUES( 1),
        ( 5),
        ( 3)
)
SELECT num, sum(num) OVER (ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW)
FROM data;

All the num added:

 num | sum 
-----+-----
   1 |   1
   5 |   6
   3 |   9

How can add all the num from bottom till top such as:

 num | sum 
-----+-----
   1 |   9
   5 |   8
   3 |   3

First of all, there needs to be a second column which provides the ordering you seem to think exists in your table. Note that SQL tables are based on unordered sets of tuples.

<!-- language: sql -->

WITH data (id, num) AS (
    VALUES ROW(1, 1),
           ROW(2, 5),
           ROW(3, 3)
)

SELECT num, SUM(num) OVER (ORDER BY id DESC) AS sum
FROM data
ORDER BY id;

Here we are taking a rolling sum in the reverse order.

<h2>Demo</h2>