Authorisation Issues

Hello,

So in Adobe Digital Editions I am trying to authorise my computer as it is saying that my item is authorised with a Vendor ID. The thing is, is that it is not allowing me to select another eBook vendor apart from Adobe ID and my ebook is not authorised with this account, its ebook vendor is booktopia.  So now i am locked out from reading my book because it is claiming that an Adobe account is attached to the book when it is not.

I hope this makes some sense and please help me!

Thank-you