While roundcube doesn't offer the function to export all email, it will usually allow you to connect a desktop mail client via IMAP with that functionality.
I recommend using an open source mail client such as Thunderbird, as it's free and well supported.
- Download and install Thunderbird
- Use IMAP to connect to your email
- Example settings:
- Wait for emails to download
- In the top menu bar, go to Tools > Export to export all emails
