The Web Interface EPG data should automatically keep itself up to date as long as your Humax is in full standby for the housekeeping process for 3am i.e. has been put into standby using the remote and is not within 15 minutes of a scheduled recording.
You can force an EPG refresh by going into the Guide pages on the Humax interface. The sound will cut out while the new EPG data is downloaded. Wait for the sound to come back and then exit the Guide pages. Reboot the box (go into Standby and then back out of Standby). Once rebooted, the Web Interface should show a message at the bottom of the EPG pages along the lines of "New EPG data has been found and is being processed". After five minutes or so the EPG data should be back up to date.