Within Letter Writer, by default when you press a key on your keyboard, characters are normally inserted into the text of a letter. This means that as characters are typed, any text to the right of the cursor is pushed further to the right. For most users this is the default and preferred mode.
In some instances it may be desirable to replace the existing text with new text, instead of simply adding the new text.
Press the Insert key on your keyboard to toggle between 'normal' and 'overtype' modes. In Overtype mode, an OVR indicator is displayed in the Status bar at the bottom of the Letter Writer window.