<insert> key to toggle inserting vs overwriting.
C-h k <insert> tells you:
<insert> runs the command
overwrite-mode, which is an interactive
compiled Lisp function in
It is bound to
(overwrite-mode &optional ARG)
Toggle Overwrite mode.
With a prefix argument
ARG, enable Overwrite mode if
positive, and disable it otherwise. If called from Lisp, enable
the mode if
ARG is omitted or
When Overwrite mode is enabled, printing characters typed in
replace existing text on a one-for-one basis, rather than pushing
it to the right. At the end of a line, such characters extend
the line. Before a tab, such characters insert until the tab is
M-x quoted-insert still inserts characters in
overwrite mode; this is supposed to make it easier to insert
characters when necessary.
See the Emacs manual, node Minor Modes.