You might to upgrade an exitsing user to able to securely connect to server using SFTP (22) protocol instead of FTP (21). Before proeeding you might want to check if your current user has "ftp-user" privlidge or not.

In order to check that you can run the following command into the ubuntu server:

groups USER_NAME

If you see "ftp-user" in the output only then you should proceed further.

Moving forward, you can ssh into your server already using root user and open the following file:

/etc/ssh/sshd_config

At the end of the file, copy & paste the follwing code:

Match User user
    ForceCommand internal-sftp
    PasswordAuthentication yes
    PermitTunnel no
    AllowAgentForwarding no
    AllowTcpForwarding no
    X11Forwarding no

Replace user with the username from above. Finally, save the configuration and restart SSH service

sudo systemctl restart ssh

That is it! You can now log into server using port 22 on the same "user" and also be able to access the Filezilla securely.