All subscription changes can be made at ultiworld.com/my-settings.
Subscription payment happens at the beginning of the period that it covers and automatically renews.
To stop your subscription from renewing, switch plan to "None" or press the red "Cancel Plan" link. The Update Subscription button will change to a Confirm Cancellation button, then make sure to finalize your choice by pressing that button.
If you downgrade your plan or set your subscription to stop renewing while in the middle of a pre-paid period, you will continue to receive all subscriber benefits through the end of that period and then the changes will take effect. This means you can safely set your subscription to downgrade or end in the future without worrying about losing your current access.
All plan upgrades take effect immediately and will be charged the difference in cost with your current plan. If you are upgrading to or from an Annual Plan (including Annual -> Annual), the difference in cost will be prorated; otherwise, it will not be prorated.