amazon web services - setting up root password in aws linux instance

we have setup the aws linux instances with ssh keys.

Ubuntu user can be login with ssh key , sudo su - getting into root user without prompt password and set the password for root even though not prompting password. how to switch the root user with password using sudo su - command.

ubuntu@ip-172-31-6-222:~$ sudo su -
root@ip-172-31-6-222:~#