Every time Avast introduces a new numerical program update, I uninstall the old and do a clean install as recommended by Bob.
Example: Old version was 12.3.3154.0 (08/23/2016), new is17.1.3394.0 (02/27/2017). This procedure is at the minimum standard, changing only to a clean install from version 12 to 17. Maximum would be a clean install at every program update: e.g., 17.3.3442.0 (03/28/2017) to 17.3.3443.0 (03/31/2017) to 17.4.3482.0 (05/04/2017), etc., depending on whether the old versions had significant bugs in it.
Otherwise updating via Avast GUI should suffice.
Save the older full executable file just in case!