Do you have a serial number? Did you ever register it? If so, you can retrieve your serial number by going to the main page of adobe.com, clicking on My Adobe at the upper right side of the page and working your way down through the links there.
You can not run trial version for more than 1 month without registering your product which in other words mean pay for serial number key.
If you wish to upgrade the price is less. Here is the link which will help you on all those details: