You can access the full version of Avast Premium Security at no cost by using the official trial options provided by Avast:

Occasionally, Avast offers a 60-day trial, though this may require you to enter payment details that will be charged only after the trial ends unless canceled.

Avast Premium Security | Online Security — Up to 10 Devices

Upgrading from the free version to adds several advanced layers of protection: Avast Free Antivirus Avast Premium Security Malware & Virus Blocking Ransomware Protection Advanced Firewall Fake Website Shield Webcam Protection Sensitive Data Shield A Note on "Free" Cracks and Keys