How do I change my user ID for my IBooks account.

And on my Mac? My MacBook used to belong to someone else. No I wanted to upload an pdf document to iBooks, thats when I realized, that on iBook there was still the previous owners Apple ID... Everywhere else it was changed, just not there. What do I do?

On an iPhone or iPad? Got to "Settings" -> "Store" and tap on the Apple ID at the bottom of that screen to change it.

In the iTunes store on a Mac, you can just sign out and sign back in again with a different ID.

Michi.