For automatic updating the feeds you will need to call the feedcron periodically. On unix style systems this is usually done using a cronjob. Most hosting providers allow to set up some cron.
There are two options calling the webpage using w g e t, curl or lynx. Or using the CLI version of php. The choice depends on the availability using the CLI version is highly recommend, this avoids max_execution_time problems.
a typical webpage request looks like:
0 4 * * * w g e t http://www.example.com/administrator/components/com_datafeeds/cron/feedcron.php
(remove the spaces in w g e t)
a typical command line call:
0 4 * * * (cd [path to the feedcron];php feedcron.php)