Here is a guide for the exchange chatbot which explain how to cancel an order:
https://docs.google.com/document/d/1n4DIR_IQwuyOn7MFNIJlxtYYbg3XAtwSzjnugHjI10w/edit?usp=drivesdk
There is also this post https://steemit.com/drugwars/@honolulu/how-to-cancel-an-order-in-obyte-https-steem-byteball-org-dropdown-toggle-poprtvg3fi7fcc3muiked6doe6vvrkl2wdh