Go to Help > System Info. It will have your serial number on top. Keep it in a safe place (the serial number is what you pay for).
Then download the latest version (4.3) here (assuming Windows): http://www.adobe.com/support/downloads/product.jsp?product=113&platfor m=Windows
Then you just enter your serial number when you install that. Alternatively you can download the trial version, but AFAIK that will be 4.0, not 4.3, so you'll have to update it anyway.