First ssh into your server using the ssh key.

ssh -i "Mysecretkey.pem" ubuntu@ec2-ip.compute-1.amazonaws.com

Once you are logged in, switch to root user with the following command

su root -

Then run passwd root

It will ask for the new password

New password: // Enter new password here
Retype new password: // Re-Enter new password here

Once everything is correct, you will get the success message

passwd: password updated successfully

Congratulations, you have successfully changed your root password.

Also, to change the password of the specific user, you can repeat the same procedure, just provide the different username with the switch user (su) command.

That's it !