There are two options for changing a redemption rate for a customer,First option would be giving that customer a special rate or the other or the next option would be

At the Home Screen you want to Select the Cash Till Transfer, like the photo below, Click the Magnifind Glass And select the till you are want to transfer out,

When your at the New Sale Screen, Enter the Stock Number you are wanting to discount Click the item you are wanting to discount, so it highlights Red, Then Click