Right click on any group, variable or expression and select Properties to access/edit the information.
If you right click and select Properties on a variable or expression you will be presented with the Variable Properties window.
If you right click and select Properties on a group you will be presented with the Group Properties window.
The Properties option is not available for the Material Variables group; neither is it available for standard variables that are linked to common variables. See Save in Library and Add from Library for more information about linking variables.