ftp for Ubuntu 16.04 Apache
SFTP over SSH is enabled on our Linux images by default, you shouldn't use FTP since it's un-encrypted and old. FileZilla is a popular desktop client to use with SFTP, just specify port 22 and your SSH credentials:
If you've made Linux users on your system for each "site", then you would just provide those login credentials:
If you're unfamiliar with all this, you'll need a Linux sysadmin to tailor the permissions and home directory for each Linux user so that the users are restricted to only the files you want them to edit.
My team, Linode Professional Services, could help with this. We're basically a sysadmin team for hire here at Linode. If you're interested, hit the "get a quote" here and fill out your information: