The big change is that the premier Access Anywhere feature has been replaced by a new Remote Control tool available via the Account screen in all editions (click Account, scroll down to My Other Devices).
Avast’s Browser Cleanup technology is now integrated with the main interface, and run as a part of the Smart Scan. If you have some “bad add-ons” then you’ll be warned by default, and have the chance to remove them.
Elsewhere, Avast’s NG virtualisation is faster and more stable, while WebShield has less impact on browsing speeds and does a better job of handling expired certificates.
Other items on the “What’s new” list include better licence activation during setup, and a redesigned Chrome plugin, and the whole package is now apparently more compatible with Windows 10 Technical Preview.
If you have Avast already installed then it’s safest to upgrade via the product’s own updater (Settings > Updates > Program).