Once you have created your banner, it can be previewed by clicking on the magnifying glass button. You can ensure that the banner is displayed correctly before publishing and sharing it to media outlets.
If your banner has been reviewed and published, and you want to make some changes, you can simply edit the banner, and use the preview to see the changes. This will not have an effect on the live version of your banner until you republish it.
Use the live preview button inside the banner preview to see if the current version of your banner looks correct.