Many users are used to editors that are normally in overwrite mode : when you backspace and start typing, you "type over" the character that is underneath the cursor.  By default, Emacs works in insertion mode , where new characters are inserted just before the cursor's position.
If you prefer overwrite mode, just give the command
If you always want to use overwrite mode, create a file named .emacs in your home directory, and put the following line in it:
(setq-default overwrite-mode t)
This is a simple Emacs customization; for a lot more about customization, see O'Reilly & Associates' Learning GNU Emacs , by Debra Cameron and Bill Rosenblatt.