How exactly do I add a key to my DigitalOcean Droplet??

I changed my public and private keys on Terminal (I’m using Mac OS) many times. So when I “ssh” into the IP address given to me by my Droplet (the IP address is 107.170.60.7), I get the error of Permission denied (publickey). so I know it has to do with me changing the public/private key. So now I need to add that changed public key on my Terminal to my Droplet on DigitalOcean. There were two things suggested by the DigitalOcean community which I’ve tried:

1. The tutorial on here - but this is for Windows users, not Mac. It’s suggests using a “Search and Replace” and checking certain functionalities. But Macs don’t have this so I can’t even use it: http://geektnt.com/how-to-add-ssh-key-to-existing-digitalocean-droplet.html

2. A user kept suggesting to someone else that the problem is resolved by visiting this page: https://www.digitalocean.com/community/tutorials/how-to-set-up-ssh-keys--2 I read into it and it has everything to do with setting up a SSH key, which I already know, and NOTHING to do with how to ADD an ssh key to an existing Droplet.

I really just need to know how to add my changed ssh key to my Droplet. I would really appreciate the help if anybody knows.