You update4 it by connecting it back to wither a phone line or ethernet connection(if applicable) and letting it connect to the TiVo mothership and get an update.
You will get everything in the software that has been updated since you last connected, not to mention programming information. This depends on your current software version.
TiVo Premiere 4 ∙ WD 1TB DVR Expander ∙ TiVo Mini ∙ TiVo Stream
XBOX LIVE: fiftyfootfang