UPDATE changes values in existing rows:
UPDATE products
SET price = 34.00
WHERE product_id = 4;
SET lists the new values, separated by commas when you change several columns. The new value can be a calculation from the row's current values, such as SET price = price * 1.10.
WHERE picks the rows to change. Without it, every row changes: the most common and most expensive mistake with UPDATE. A safe habit at work is to run the WHERE as a SELECT first and check that it returns exactly the rows you mean to change.
The check compares the table after your script with the table after the answer, so a missing or wrong WHERE shows up as changed rows that should have stayed the same.