There should be a drop down box at the top that says Reported Period and you just pick the old months/years. I have data all the way back to May 2003 for one domain.
If that flat out is not an option for you when viewing AWStats then you must have turned off a standard feature. Check the awstats.conf file for the available options.
[color=#CC0000]Custom AWStats Installed, Password Protected and Configured for Hourly Auto Update - $20 (PM with inquiries)[/color]