Hit the <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 simple.el.
It is bound to insertchar, insert.
(overwrite-mode &optional ARG)
Toggle Overwrite mode.
With a prefix argument ARG, enable Overwrite mode if ARG is
positive, and disable it otherwise. If called from Lisp, enable
the mode if ARG is omitted or nil.
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
filled in. 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.