To delete your leave requests, just type delete to the bot and all your upcoming leaves will be displayed. Each leave will contain a Delete option right below it.
You can then press the Delete button for the particular leave which you want to delete and the bot will ask you to confirm the action.
Once you confirm this(by pressing the Delete button), that leave will be deleted and you will be notified of the same.
Note: If you want to delete a past leave request, you need to ask your manager to do so. The process for your manager to delete your past leaves is explained in this article.