To unregister from a tournament you're already registered to play in, firstly locate the specific tournament. To do this, log in to your account and from the main lobby screen, select:
Once you've located the desired event, simply select the Unregister button.
You can also use the hotkey Ctrl + R (press Ctrl and R on your keyboard at the same time) to see the Registered in Tournaments report.
You cannot unregister from a tournament before its registration opens.
If you can unregister, your account will be credited in the same way you paid the buy-in.