Once a transaction has been set as "Confirmed", it cannot be reverted anymore, thus it cannot be canceled.
Before a transaction gets the above-mentioned status, so it is still "Pending", because of low gas or other issues, you can do the following steps to revert/cancel the transaction:
- Do a soft reset of the application, Go into the "More" menu, "Reset application" leaving only the first option checked hit "Reset".
- After the reset, do a 0 high gas transaction to yourself. You can change the amount of gas from Normal to High in the advanced options, top left corner of the transaction window.