Insert and Overtype Mode
Typically, you edit a document using Insert mode. This means text to the right of the insertion point moves to the right as you type new text. Insert mode adds text, but it does not erase anything.
Press the Insert key (located to the right of the Backspace key) once; you can change your computer to Overtype mode.
When your computer is in Overtype mode, text you type replaces any existing text to the right of the insertion point and erases it.
You can tell which mode you are in by looking at the Status bar at the bottom of the screen.