Firstly, our AMIMOTO AMI does not accept ssh access as a root user:
$ ssh -i path-to-key-file root@IP-Address
Please login as the user "ec2-user" rather than the user "root".
So, you should access to the instance with ec2-user, like:
$ ssh -i path-to-key-file ec2-user@IP-Address
Then type sudo su - and hit return key
[ec2-user@ip-xxx-xxx-xxx-xxx ~]$ sudo su -
The prompt has changed from $ to #.
When you think logout root user and becoming ec2-user, simply type exit then hit the return key. The prompt changes from # into $.
[root@ip-xxx-xxx-xxx-xxx ~]# exit