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If you press the "Insert" key or "Ins" (located to the right of the Backspace key) once; you can change your computer to Overtype mode. When your computer is in Overtype mode, the text you type replaces any existing text to the right of the insertion point and erases it.
Press the "Ins" key to toggle overtype mode off. Depending on your keyboard model, this key may also be labeled "Insert." If you simply want to disable overtype mode but keep the ability to toggle it back on, you are done.