I don't know if this is true for you or not, but I always forget how to change my store banner. I clearly don't do it often enough. LOL.
In case you have the same issue as me, here's a quick How-To...
First, you need to be on your dashboard. From your dashboard, click on the Marketing tab.
On the Marketing tab, click on the Customize option to add / change the top and side banners in your store. You can also attach a web address from within TPT to the banners.
Easy peasy, lemon squeeze...
Hope writing this out will help me remember,
but maybe I can help someone else, too!