或許取決於你用的瀏覽器？

“...That depends on the browser you use, i.e. a browser like Google Chrome (I just tested) and FireFox (I believe) do not support an insert/overwrite toggle. But a browser like Internet Explorer does and uses the Insert button for that.”

http://www.google.com/support/forum/p/Google%20Docs/thread?tid=11211bc996704407&hl=en