The Layers Panel is needed for managing the layers. It shows the list of names of layers ordered by their location from top to bottom. The layer in bold is the one currently selected for editing. In order to select another layer, click on its name on the Layers Panel.
A context menu is also available for managing the layers.
PixBuilder Studio allows selecting several layers at once by holding down the Shift key while clicking on the desired layer names. You can also do this by encircling the desired layer names with a mouse.