Yes, you can update your unsubscribe message.
Go to Settings Tab by clicking on the gear icon located on the top right of your page. On the left pane, click on Unsubscribe message. Now you will see your present unsubscribed message there and against it is a Action column in which you will see the pencil icon. Click on it and a popup will be poped where your previous message will be displayed and there you can edit / change your message in the was you want.