For Mac OS 9, Apple stopped upgrading iTunes at version 2.0.4. If you need a copy, type iTunes 2.0.4 in the Knowledge Base search box at www.apple.com/support to find the page with the Read Me file and download link.
If you use iTunes for Windows, you can tell the program to alert you to newly available versions by choosing Edit → Preferences → General and turning on "Check for iTunes updates automatically." If you prefer to check yourself, choose Help → Check for iTunes Updates or visit www.apple.com/itunes/download.