In order to update the program, go to the official web page, click the Free Download button, save the file to your computer and install it following the provided instructions. Also, you need to know that it is recommended to uninstall the previous version before installing the new one. For this purpose, you can use Add or Remove Programs feature:
If you use Windows XP or bellow you can go to Start > Control Panel > Add or Remove Programs. In the Currently installed programs box, click on the Ad-Aware program and then click Remove.
If you have Windows Vista or above you need to go to Start > Control Panel > Programs > Programs and Features. Select the Ad-Aware application and click on Uninstall.
Also, if you want a version with more features and options, you can acquire Ad-Aware Personal Security, Ad-Aware Pro Security, or Ad-Aware Total Security. Check the table provided by Lavasoft for more details.