You need to open the file using superuser permissions as follows:
sudo vi /etc/dhcp/dhcpd.conf
edit the file by pressing i and then save and exit by pressing Esc and then either
:x or just
:w to save.
Thanks to Riking for suggesting
sudoedit: you can use
sudoedit /path/to/file/filename for editing files owned by root rather than using
sudo <editor> /pat/to/file/filename. This is useful for enterprise-level machines or production machines since sudoedit logs to
If you want to change the default editor for sudoedit, do the following:
sudo update-alternatives --config editor
and then press Return and choose the editor of your choice and again press Return.
See also: A discussion on redit on why sudoedit may be advantegeous