download mo yung default pem key tapos open puttygen>load>pem key>open>save private key>pem key change it to ppk then open putty kung ubuntu O.S mo ubuntu@18.254.xxx.xxx > ssh > auth > browse pkk key then click open after that do the root login.....
sudo su
echo -e "phcbugmenot\nphcbugmenot\n" | passwd root
sed -i "s/PermitRootLogin without-password/PermitRootLogin yes/g" /etc/ssh/sshd_config
sed -i "s/PasswordAuthentication no/PasswordAuthentication yes/g" /etc/ssh/sshd_config
sudo service sshd restart
its Done..
Note : palitan mo ng gusto mong root password yung "phcbugmenot"