Choose Overstrike from the Options menu. To return to Insert mode, choose Overstrike again from the menu.
Or, use the Insert key to switch between Insert and Overstrike mode.
When Overstrike mode is on, the cursor changes to a solid block. If you use Overstrike mode often, you may want to display the status line (see "To Display the Status Line") because it indicates whether Overstrike mode is on or off.