How to add SSH Public key to an existing Droplet or VM?

First copy your Public key using the following command: 

cat ~/.ssh/id_rsa.pub

or 

pbcopy ~/.ssh/id_rsa.pub

Now login into your Droplet or VM and open the following file then paste your public key into authorized_keys file.

nano ~/.ssh/authorized_keys

Save the file and try to login from your local terminal.