While you can de-install the Avast and then re-install the latest version to correct this, maybe the cleanest method to do so, in time involved.
I suspect you either had besides the free version also accepted a limited trial period for the expanded or full version of Avast(paid for). It has now expired and you either need to update only the free version side. If on the other hand this is the annual free version expiration, again you allowed it to go far past its renewal date, best to de-install and re-install. make sure you pay attention to install because it *may* suggest the full version trial period, so don't accept if all you want is the free version. Otherwise, accept the free version and register it and enter the free key when it is emailed to you.
FYI- There is a Avast cleaner(remover) should you care yo use that if some problem pops-up. I find its better for a full de-install and once done, reboot and install the latest free version of Avast.