Avast has a rull few parts that can be updated:
1) Program itself
2) VirusDataBase
You can to manual or set to automatical updtae in the avast settings.
1) Right click on avast icon in the tray -> chose "Updating" ->> now you can see few menu
1.1 Program Update
1.2 iAVS Update
Chose first "Program Update" after update is completed chose "iAVS Update"
Reboot Computer if avast asking, in order to complete update.