Change Username

Enable Root Login

passwd root
sed -re 's/^#?(PermitRootLogin .*)/#\1\nPermitRootLogin yes # delete-me/' -i /etc/ssh/sshd_config

Change Username and Password

Login as root, and do the following:

sed -re "s/^pi /${NEW_USER} /" -i /etc/sudoers.d/010_pi-nopasswd

mv /home/pi "/home/${NEW_USER}"
usermod pi -l "${NEW_USER}" -d "/home/${NEW_USER}"
groupmod pi -n "${NEW_USER}"
passwd "${NEW_USER}"

Logout and Login as ${NEW_USER}.

Disable Root Login

usermod root -p '*'
sed -re '/^PermitRootLogin .+ # delete-me$/d' -i /etc/ssh/sshd_config