Shelley Hitz explains how to publish a book bundle on Kindle – though it would be the same for any site you sell on. Basically, it’s putting all the books in one file and making a new image (like a box set) for it. Pretty simple, I guess. But unfortunately, she doesn’t say how to make the new box set image and I’m not so good with photo shop. I guess I have to ask some friends to help me. There are lots of people that are good with photo shop.
While you’re waiting for help with your new cover, make sure your series ebooks are connected. I know Amazon has a specific line that you put the same series name in for each book in the series (mine is “Rose series”) then the volume #. They are supposed to be displayed together when someone pulls up one book in the series. I know you can do that on Goodreads, too.