Login to ACP, select the database, then on the database dashboard, click the version widget and ACP will prompt you for which version you want to upgrade to. Only after you select a version, will ACP detect that there is already a pending upgrade, and ask if you would like to cancel the upgrade. Click yes, then close the window. Do not continue to schedule a new upgrade. Note you can only cancel upgrades that have not started yet. You will have to contact support if you wish to roll back to a version prior to the upgrade. All data changes since the upgrade will be lost in the roll back.