I had problems updating it last week (due to Avira's servers been over burdened by a big update) but I am updating fine this week.
This sometimes works; go into Avira program folder this is usually,
C:Program files/Avira/AntiVir PersonalEdition Classic
In this folder you will find a file called "preupd" double click on this. If this won't let you update then it may be that your firewall has blocked it, check in your firewall for programs/files that are blocked by it, look for an entry called "preupd" or any others that have the name Avira. :-)