You can cancel or pause your promotion it by setting it to Draft status, while you make any changes needed.
To do this please log in to Jungle Scout and click on the Launch option, then go to your Promotions page. From there, click on the Published button of the promotion you'd like to pause/cancel. As you hover over it, it will turn gray:
Once you're in the edit page for that promotion, look for the Actions... menu at the top right side of the page and click on it to see the dropdown menu. You will have the options to Save as Draft and Cancel Promotion:
After that, your promotion will be unavailable to shoppers.