Click Edit on the post. Once the editable page opens, on the top right corner there is a ADD COVER IMAGE button. Click on that. Then click Remove Cover and Upload/select a new cover image. Hope this helps!
Hey, You can only change the over page of your blog on your web page of Tripoto and not on app version ... Log into in your desktop or laptop and then update it ... and always Blogging in your web version is the best as you can take care of everything at once in your web version.... Happy Blogging!!!