Inverting a selection will reverse the features that are selected, either with the Digitizer tool or in the Attribute Editor. This option is available in the digitizer tool when vector features are selected.
To invert the current selection, right-click or go to the top-level Digitizer Menu to access this tool from the Advanced Selection Options submenu.
This tool will deselect any visible vector features that are selected, and select any visible vector features that are not selected.
This option is also available in Search tool. Right click on one of the selected features in the search results to see the option to invert the selection. This will deselect features selected in the search results, and select any unselected features in the search result.