Any time after publishing the article you can edit or delete the cover image of the article. Very simple and easy steps. Lets see a overview.
If you want to edit or delete the cover image of an article you just need to go to the edit page first. Then by clicking on the delete button/ Bin button at the bottom middle of the existing cover you can remove/ delete the cover image of the article.
Now the edit procedure is -- for edit as well you first need to delete the existing cover and then again click on the Upload cover image section to browse your local computer and just select one image and click on OK. Your new cover is ready to publish.