if you have no internet connection you cant update to version 4.8 1290 ;D by the time you have your internet connection you can update by right click the avast ball icon then left click updating then left click program updates OK
of course you can update - if you have downloaded the new installation file, you simply have to install the new version over the old one!
so keep your current installation and doubleclick the new installation file - then you have the possibility to choose something like update!