update positive to negative value in mysql

If I understand you correctly, you also want to set amount column to positive value along with the above statement.

You can use something like this

UPDATE payment 
SET reason = 'refund', amount = amount * -1, total = total * -1
WHERE uid =5 AND date = '2012-05-01' AND accid =2

Use ABS(amount) if you wish to always get the positive integer.

SELECT ABS(5);

will output 5

SELECT ABS(-5);  

will also output 5