Somehow I haveflipped a switch enabling overtyping (?). Instead of inserting a character the new replaces the existing. How do I change this?
Usually this just means you tapped the "Ins" (insert) key on your keyboard, which toggles between Insert and Typeover. The Application Bar at the bottom of your display will tell you if you are in Insert or Typeover mode. You can also change to Insert mode by clicking there.