I've created several items now I would like to change the author details.
Which "Author" are you refering to?
The Joomla user who created the item can be changed by clicking on the author name. A list of all available Joomla users will pop up and you can select a different one.
Or are you refering to the ZOO item of the type "Author"? The "Author" in the ZOO Blog app is usually assigned via a ZOO Related Items element.
From within an "Article" item you can only select from the existing "Author" items but you can neither create nor change the content of an "Author" item.
To edit an "Author" item's details edit the "Author" item.