How do I read books that were downloaded on old account with a new one?

When I try to download the books I get this error message: "The vendor account you entered is not associated with the item you are trying to open.  Try again"  I had same problem few months back but then I got help from somebody to merge the accounts and I cannot remember how to do it.  I´m trying to download the books to my ipad (which I use for reading the books) and there it always says: "This application is authorized with the following user account: then my new account    This item was originally downloaded with a different user account or Adobe ID.  To continue, please add that account".