To change your sender name in the Roundcube webmail, log in to your webmail via https://yourdomain.com/webmail (replace yourdomain.com with your actual domain name) or via your Skystra Client Area, as explained in this article.
Once in the Roundcube webmail, click the Settings option, and select the Identities submenu:
You can then edit the existing identity by clicking on it or create new identities:
That’s it!