How do I get it?
Do you know that avast has a FAQ section? https://support.avast.com/
Avast Secure Browser - FAQs https://support.avast.com/en-gb/article/Secure-Browser-FAQ
Try checking/browsing the Avast Secure Browser sub-forum.
Have you tried a search, I did and found this https://support.avast.com/en-gb/article/Use-Secure-Browser and this https://www.avast.com/en-gb/secure-browser#pc.
This may also help. https://youtu.be/H5IMQEeU6Ho