Hi, Rebekah,

Just press the Insert key to toggle between "insert mode" and "overtype
mode". The indicator is the letters OVR on the status bar at the bottom of
the window -- if the letters are gray, you're in insert mode; if they're
black, you're in overtype mode. (You can also toggle the mode by
double-clicking the OVR letters.)

If you find that you often hit the Insert key by mistake, you can go to
Tools > Options > Edit and check the box labeled "Use the INS key for
paste". That will change the function of the key so that it pastes the
clipboard contents (if any). You can still toggle overtype by
double-clicking the OVR indicator.

Tools | Options | Edit: Check the box for "Typing replaces selection."

--
Suzanne S. Barnhill
Microsoft MVP (Word)