Once you have logged into RoundCube, click on the 'Settings' link next to the wheel at the top right corner. Click on 'Identities' along the left side and then click on the email address in the center column. In the right column you can update your name and organization information. You can also add a "BCC" (blind carbon copy) email address which will copy that person on all of the emails you send.
In the 'Identities' section you can also add or update your email signature. This will add your email signature to every email you send using RoundCube. When checking 'HTML signature' this will allow you to write HTML code to customize your signature.