Where do I change how text is selected/replaced text when editing?

[Jaylee]

I've clearly changed a setting in word without knowing it...usually I can
select text that I want to change and when I begin typing, the selected text
is automatically deleted/replaced with the new text. Somehow I've changed
that setting...when I select text now and start typing, the selection is
immediately unselected and the new text is simply inserted in front of the
text I'm trying to replace. (closing the program and even rebooting did not
reset this setting.)

I know there is a simple option or formatting solution to this, but I've
been unable to find it. Thanks.

 

[Suzanne S. Barnhill]

Tools | Options | Edit: Typing replaces selection.