Hi Stewart
It will ask for a pc restart to complete the install so if you are not getting that far then it wont have installed the latest update.
I think you need to give it more time than several minutes.
Is the update still showing in red. no idea if this will help but click on more options and set to I’ll update manually then click the install again and go and make a cuppa. if after 10-15 mins its still not installed contact support
Avast should also tell you that program version is now also outdated, as 25.2 has since been released. (unless you are holding on to 25.1 on purpose for some reason).