If you are using the free version of AVG, then there is no need to input a license key. The software will automatically assign one based on the registration that you have to fill within the software. I know that AVG features a way to reactivate the software from its interface. Therefore, go to the Options menu and look for the activation section, then simply renew the activation if it expired.
Can't say the same if you're using the PRO version. Once the trial expires, it will prompt you to enter the serial number that needs to be purchased from the developers' website.