The Browse Libraries dialog allows you to browse schematic symbols in selected libraries.
The dialog is accessed from the Variant Management dialog (Project » Variants) by right-clicking in the Component Parameters region, then selecting Update Values From Library.
The Browse Libraries dialog only appears if the currently selected component in the Variant Management dialog is from a file-based library, or is a managed component and you are not currently signed in to the parent server from which it was placed. If the selected component is a managed component, and you are signed in to its parent server, then you will be presented with a dialog that is an incarnation of the Explorer panel - enabling you to browse the components within that server.
Libraries - displays the current library. Use the drop-down arrow to access a list of available libraries then select the desired target library. Click the button to open the Available File-based Libraries dialog in which you can add or remove libraries.