How to subtract a number from a value in a MYSQL table?

Question

MySQL

How to subtract from the value of amount (in the table) the number $sum?
I tried that - didn't work

$sum = 1;

$mysqli->query("UPDATE orders SET redeemed = '$change',  amount -= '$sum' WHERE id = '$id'");

Answer 1

UPDATE orders SET redeemed = '$change',  amount = amount - '$sum' WHERE id = '$id'