How to test run an UPDATE statement in PostgreSQL?

Use a transaction to wrap your update statement and a select query (to test the update) and then always roll it back.

Example:

BEGIN;

UPDATE accounts SET balance = balance - 100.00
    WHERE name="Alice";

SELECT balance FROM accounts WHERE name="Alice";

ROLLBACK; -- << Important! Un-does your UPDATE statement above!

A transaction typically ends with a commit but since you’re just testing and do not want the changes to be permanent you will just roll back.