The update from 4.0 to 4.1 should be handled just as any other program update - just select "Program update", either from the Simple User Interface menu or using the avast! icon in the system tray, and the update should be downloaded and installed.
The registration key is the same for 4.0 and 4.1 - so if your key is still valid for 4.0, it will be valid for 4.1 as well. If the key is already expired (i.e. it's more that 14 monts old), you can simply register again and receive a new key.