Installing or Update SSH public key for Authentication in a existing Linux VM

I have Linux VM that are provisioned with a ssh key for authentication. As per instructions here:

http://www.windowsazure.com/en-us/manage/linux/how-to-guides/ssh-into-linux/

They work fine. However I also have Linux VMs that have been created without a ssh key, which means I need to type in the password everytime I ssh into these VMs. 

My question is, is it possible to install a public key for authentication after the VM has been provisioned. Or update the key that is already installed?

thanks