To use а SSH key pair to connect to a server, the "public" part of the pair has to be placed on the server. Please refer to the first section of the "How to create and access a Linux-based cloud server via an SSH key" article for instruction on how to generate key pairs under Windows, Mac, and Linux.
Now, when you have a key pair, you need to add a public key to the "authorized_keys" file in your home directory on the server.
1. Connect to the server using your current key
ssh email@example.com -i /current_private_ssh_key
2. Add a public key to the "authorized_keys" file
You can add a public key to the "authorized_keys" file using "nano" text editor (or any text editor of your choice):
Paste your public key from the new line:
Save changes by pressing Ctrl+X, Y, Enter.
If you don't have "nano" installed
You can install "nano" by running the following commands on Ubuntu:
sudo apt update && sudo apt upgrade sudo apt install nano
yum update yum install nano -y
3. Verify new key
Do not terminate existing SSH session. Open new session with the key you have just added:
ssh firstname.lastname@example.org -i /new_private_ssh_key
If the connection is established successfully, you all right. To remove one of the keys, just remove it from "authorized_keys" using the same "nano".