How do I revert/cancel a transaction?

Once a transaction has been set as "Confirmed", it cannot be reverted anymore, thus it cannot be cancelled.
Before a transaction gets the above mentioned status, you can go under "Settings" and use the "Reset Nonce" feature, after that you will have to execute again a transaction to be able to revert the previous one.
Note that you can also do a "Null" transaction by sending 0 value of the selected currency to yourself.