Debian 10

For each of the following methods, make sure you have the right credentials to each remote server you are working. You need the right pair of username and password of each machine for a successful transfer.

Scenario
Your backend is accessible only through ssh key-pairing but you have the need to use an FTP program like FileZilla to transfer a file from or to a local machine.

Temporarily disable the ssh only access
From the command line:

1. sudo nano /etc/ssh/sshd_config
2. Set PasswordAuthentication to yes
3. sudo service sshd restart

From your FTP application

Certbot will let you manage your SSL certificates, ensuring that your server will automatically renew Let's Encrypt without you doing it by hand.