Ways to change Ec2 instance key when you lost or corrupt origional key

First Way :
Follow the below steps 

Instruction from AWS EC2 support:

1. Change pem login
2. go to your EC2 Console
3. Under NETWORK & SECURITY, click on Key Pair Click on Create Key Pair
4. Give your new key pair a name, save the .pem file. The name of the key pair will be used to connect to your instance
5. Create SSH connection to your instance and keep it open
6. in PuttyGen, click "Load" to load your .pem file
7. Keep the SSH-2 RSA radio button checked. Click on "Save private key" You'll get pop-up window warning, click "Yes”
8. click on "Save public key" as well, so to generate the public key. This is the public key that we're going to copy across to your current instance
9. Save the public key with the new key pair name and with the extension .pub
10. Open the public key content in a notepad
11. copy the content below "Comment: "imported-openssh-key" and before "---- END SSH2 PUBLIC KEY ----
    Note - you need to copy the content as one line - delete all new lines
12. on your connected instance, open your authorized_keys file using the tool vi. Run the following command: vi .ssh/authorized_keys you should see the original public key in the file also
13. move your cursor on the file to the end of your first public key content :type "i" for insert
14. on the new line, type "ssh-rsa" and add a space before you paste the content of the public key , space, and the name of the .pem file (without the .pem) Note - you should get a line with the same format as the previous line
15. press the Esc key, and then type :wq!

this will save the updated authorized_keys file

now try open a new SSH session to your instance using your new key pai

When you've confirmed you're able to SSH into the instance using the new key pair, u can vi .ssh/authorized_key and delete the old key.

Second Ways 

1. Stop the running EC2 instance
2. Detach its /dev/xvda1 volume (let's call it volume A) - see here
3. Start new t1.micro EC2 instance, using my new key pair. Make sure you create it in the same subnet, otherwise you will have to terminate the instance and create it again. - see here
4. Attach volume A to the new micro instance, as /dev/xvdf (or /dev/sdf)
5. SSH to the new micro instance and mount volume A to /mnt/tmp
   $ sudo mount /dev/xvdf1 /mnt/tmp
6. Copy ~/.ssh/authorized_keys to /mnt/tmp/home/ubuntu/.ssh/authorized_keys
7. Logout
8. Terminate micro instance
9. Detach volume A from it
10. Attach volume A back to the main instance as /dev/xvda
11. Start the main instance
12. Login as before, using your new .pem file

That's it.

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