To change the hostname of a machine you can use the following command: "hostname mynewhostname". - With out the quotes. Worth noting is this does not affect the DNS and this is only for the current session. By rebooting the server, this information will reset to whatever Amazon machine is configured to.
If you want to make the hostname permanent then you need to edit the network file.
If you are using Linux: this command
sudo vim /etc/sysconfig/network, of course you can use what ever editor you like. In this file there is an attribute called HOSTNAME that you need to change.
If you are using Ubuntu: this command
sudo vim /etc/hostname and replace the string of text to your new hostname.