And how can I custom install a new version without uninstalling the current one?
Yaron... don't update via the GUI. Download the installation files from Avast, and run it (without uninstalling your old version). On the very first screen, in smaller print, you will find the words "Custom Installation". Click on that, and follow the screens from there, deselecting any aspect of Avast that you do not wish to install. This will do a Custom Install over top of your older version, installing as you requested, and maintaining all of your old settings from the previous version.
Gregg