AWS - EC2 - Change PEM Key

Before starting, save on your instance the key-first-one.pem ( orginal pem file which you get it at configure of the instance first time )

1. From the AWS EC2 Console -> Key Pairs and generate a new key pair (eg: keynew.pem);
Download the generated pem key, and chmod it to 0666

2. Use the new pem key to generate a public key
 $ ssh-keygen -y

 When prompted, enter the path to keynew.pem ( /path/to/keynew.pem ) . This will generate on console not in file
 Touch keynew.pub and paste his content form generated process.

 Tip: Edit keynew.pub and append the key's name so you can identify it easier in the next steps.
 Just append a single space, then a tag/name for the key

3. Add the new public key to your instance

   $ cat /path/to/keynew.pub | ssh -i /path/to/key-firstone.pem user@ip-private-from-aws "cat >> .ssh/authorized_keys"

4. Test the new key by logging in and removing the old/original key

 $ ssh -i /path/to/keynew.pem user@ip-private-from-aws

 $ nano ~/.ssh/authorized_keys

 Find the line containing the "original name" key and remove it.