When you are editing text in overtype mode in a Word document, typing new characters replaces any existing characters to the right side of the insertion point.
Turn on overtype mode
On the Word menu, click Preferences.
Under Authoring and Proofing Tools, click Edit, and then in the Edit dialog box, select Replace existing text as you type (Overtype mode).
Note: To turn off overtype mode, clear the check box next to Replace existing text as you type (Overtype mode).
Type over existing text
In your document, click where you want to type over the existing text and begin typing. The new characters you type replace the characters to the right.