If your transaction wasn’t approved by the network and the transaction hash wasn’t assigned yet, it can be cancelled.
Open the transaction page and click ‘Cancel’.
You'll see a pop-up message — once you confirm it, your transaction will be cancelled.
You can reopen the transaction page and check the transaction status: on the bottom of the page you’ll see ‘Error’ field below it ‘Cancelled transaction’.
Please note that if a transaction has been successfully completed and approved by the network, it cannot be reversed or cancelled.