After you login to your account, go to "Account Settings" and then click "Terminate Account".
You may terminate your agreement at any time; however, any money already paid will be non-refundable, as your Paperturn account will continue until the end of the paid period. Nevertheless, you will receive no further bills subsequent to this.
When you terminate your subscription, you are canceling the automatic payments from your account. Your publications will stay online until the expiration date of your paid subscription. This means that you will not receive a pro-rated refund. After that all your info will be deleted.
Yes you can. Upgrading to a more expensive plan will result in a pro-rated charge, according to the cost of your new plan.