Renaming a document or folder is possible through a variety of means within Goodnotes 6, whether you want to change the name of an existing Notebook or you've changed the contents of a folder and want the title to reflect that.
To rename a document or folder, please tap on the Down arrow to the right of the title in the library view to bring up more options. Then tap the title field to edit the name.
Tip: You can also rename a document while working on it by tapping its title at the top!