If you are already in a trade you can cancel it with the central button present in the trade GUI
Cancel Button
2 - Set the price
Price button
Clicking on the gold nugget will open an Anvil UI, allowing you to adjust your price.
The button item is configurable -> Set or Get GUI Items
Anvil UI
3 - Confirm trade
Click on the red button to confirm your trade
If you changed your mind after initially clicking the button to start the timer, there's no need to worry. You can simply click the button a second time to halt the timer
4 - Pick up your items
When the timer stops, you will be able to collect items from the other side of the trade GUI