With the keyboard:
Press Insert to toggle between overwrite mode and insert mode. The current mode is displayed on the Status Bar. The text cursor must be enabled in the cell or in the input line.
With the mouse:
On the Status Bar, click on the area indicating the current mode in order to switch to the other mode:
Insert mode is enabled. The text cursor is a blinking vertical line. Click on the area to enable the overwrite mode.
The overwrite mode is enabled. The text cursor is a blinking block. Click on the area to enable insert mode.