Please be aware that
whenever you cancel an upgrade you will only be refunded half of the cost. Once an upgrade has been cancelled, there is no way to undo the cancellation or to get a refund. Please carefully consider if you really want to cancel an ongoing upgrade before following the instructions below.
Also, note that when you cancel an upgrade, you will be refunded half of the cost in resources,
never in gems. This is the case even if you used gems to start the upgrade.
Locate your current upgrade and tap on "Cancel"
Confirm the cancellation tapping on "Okay". Again, please note that only 50% of the resources that you have spent to make the upgrade will be refunded.