To quickly access sub-libraries, click on the library name under Manage Images in the right sidebar to expand the library, showing its sub-libraries. If there is not a caret icon next to the library name, that library has no sub-libraries.
To move an entire library under another library, click Actions for that library, then click Move to…
You’ll now be able to change the parent library. To make the move, click Move.
Here, you’ll be able to edit the Image Library name, add or edit the Description, or move it either to the top-level, or under a different library.