how to edit readonly file with vi

I'm following this tutorial for deploying a node project on an EC2 instance. I have to install npm and here what I have to do:

sudo su
vi /etc/sudoers

edit by pressing "i"

Defaults    secure_path = /sbin:/bin:/usr/sbin:/usr/bin:/usr/local/bin

In order to save your changes, press ESC key and type “wq” and hit ENTER in VI editor. But I can't because when I press "i" I got an error

W10: Warning: Changing a readonly file

How do I solve this?