It depends on what you're trying to do:
1. If you want to toggle between Insert and Overtype modes, press INS,
double-click OVR on the status bar, or change the setting on the Edit tab of
Tools | Options.
2. If you want to format text as Strikethrough, select this property in the
Format | Font dialog.
3. If you want to print one character over another, look in Word's Help for
information about the EQ field with \o (overstrike) switch.
