It's documented in the readme.txt supplied with the firmware. Extract of the relevant section attached below:
TinyFTP
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The default login name and password is "HumaxFTP" and "0000" respectively.
Login credentials can be changed by editing the file "tinyftp.conf" which can be found in "/opt/etc"
vsFTPd
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If you install vsftpd instead of tinyftp then system accounts are used for login.
There are two accounts available and you can login with either.
Please note that names and passwords are case sensitive.
The first account mirrors that of TinyFTP.
User name: 'HumaxFTP' Password '0000'
Logging in with these credentials will drop you directly into the /media folder.
More experienced users may want to login using the root account.
User name 'root' Password 'root'
This opens in the systems root folder, so you will need to navigate to /media to access your recordings.
Passwords for the two system accounts only (not TinyFTP username/password) can be changed by issuing a 'passwd' command via a telnet session.
i.e. enter 'passwd HumaxFTP' or 'passwd root' and follow the prompts.
Note:
Installing vsFTPd will stop any previous installed TinyFTP's process, and disable its auto-start, and vice-versa.
This is because you can only run one FTP server at once. However, it does not remove the application.
Users should then manage the running ftp server via the service utility.