I have a user with the free version but is trying "SSL Implicit Encryption" and gets an error that an upgrade needs to be purchased. I am having problems finding where this can be purchased.
FTP Voyager is a free product which means you don't have to actually purchase it. Instead, apply an update by installing the latest version from the official website: http://www.serv-u.com/ftpvoyager
If you still receive the same issue, switch to another client called FileZilla Client: https://filezilla-project.org/