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 stop your subscription in the middle of your pre-paid period, you will continue to receive all subscriber benefits through the end of that period. All other plan upgrades/downgrades take effect immediately and will be prorated with regards to your current paid period.