avast_free_mac_security.dmgwhere you saved it in step 1., and double-click to run the file.
During the installation process, Avast may update the virus definitions, so opening up the application after installing it to update the virus definitions may not be necessary. However, you may need to update the Virus Definitions manually. To do this, please follow these steps:
Open Avast Free Mac Security from your Applications folder.