Select Option on the Tools menu and click the Edit tab. To replace existing text as you type, one character at a time, select the Overtype mode check box. Alternatively, press the Insert key on the keyboard to toggle Overtype mode. When it is on the OVR legend is not greyed out. On the Edit tab of Options you can also choose whether to enable Replace selection, Drag and drop editing, Automatically select whole word, Use the Ins key for paste, etc. Have a look at the other options, and try them out.