After logging into the admin site (click here if you are unsure how to do this), click the "Transactions" button on the left-hand menu like this:
Next up, click the "Transaction Forms" button, as shown here:
Next, find the transaction form that you wish to change and click on the "Edit" button.
You'll find the notification options on the "Notifications" tab:
Once you've made all of the changes you want, just hit the "Save" button and your preferences will be saved.