Overview
Roundcube Webmail does not include a full mailbox email export option by default
Instructions
You can usually export individual emails by opening the message and using the More / Three dots menu, then selecting Download (.eml), if available.
This saves the selected message only.
For a full mailbox export, the best option is usually to use an email client such as Outlook, Thunderbird, or Apple Mail connected via IMAP, then export or archive the mailbox from there.