Documentation | Help File | UI | 2D Mode | Menu | Select | Invert Selection
Clicking on the Select/Invert Selection menu item inverts the current selection, i.e. selects all not selected shapes and deselects all selected shapes in the currently active vector layer.
To invert the current selection on the active vector layer, click on the Select/Invert Selection menu item.
Invert Selection item in the Map area context menu.Attributes tab of the Selected panel.