The first thing to do is to make sure that the Insert key has not been accidentally pressed on the computer. This will cause text to be appended to the current text rather than replacing the text. Go into Word and try it again but this time try pressing the Insert key on your keyboard once and then see if it changes anything. You won’t see an indication on the keyboard but Word will display OVR in the status bar on the bottom when its activated.
Next you can go into your Word options and make sure that the Typing Replaces Selection option is checked. Go to File, Options and then the Advanced. Make sure the Typing Replaces Selection box is checked. The Typing Replaces Selection option deletes the selected text when you begin to type. If you clear this check box, Word inserts new text in front of the selected text and does not delete the selected text.