I am using Avira Antivirus free edition from a long time now, it is one the best antivirus of which the free version is so effective and useful, however the paid version is better but I never upgraded to premium version as I never felt the need to do so. Similarly like me there are many people who use the free version of Avira Antivirus but the only annoying thing with free version of Avira is that everyday atleast once it will pop up a windows which will show advertisements and ask you to upgrade to premium version.
Everytime you will need to close the nag pop up windows by clicking the ok button at the bottom center of the windows or by clicking the close button the window itself, as shown in the image below.
  
In order to remove this update nag screen from free version of Avira Antivirus, you can use a free program called NoNotifyAvira which allows you to remove this update nag screen very easily by adding a registry entry, follow the step by step procedure below to use this utility.

1. Download and Run NoNotifyAvira program executable
2. It will show you the command prompt, and you will need to type 1,2  per the Avira Antivirus version installed on your computer.

3. In order to check the version, open Avira Antivirus interface by double clicking the system tray icon and then go to File Menu >> Help >> About to check the version.
  
4. Now Enter the number as per your Avira Antivirus version and now it will warn you as it is adding a registry key which will stop the Avira Antivirus update nag screen, click yes button on the prompt to continue.
 
5. Now it will prompt you that it has successfully added the registry key to block the update nagging screen.

6. That’s it – now you wont see any nag screen appearing again.
Manual Method Of Adding Registry
You can also do this manually by following copy pasting the code below in notepad and then saving it as .reg file.
For Avira Antivir Classic 8
[HKEY_LOCAL_MACHINE \ SOFTWARE \ Microsoft \ Windows \ CurrentVersion \ Run]
"ATICCC" = "C: \ Program Files \ Avira \ AntiVir PersonalEdition Classic \ avgnt.exe \" / min / nosplash "
You can add the voice / nosplash that allows you to hide the splash screen antivir, On some versions you may have to write-nosplash.
[HKEY_LOCAL_MACHINE \ SOFTWARE \ Policies \ Microsoft \ Windows \ Safer \ CodeIdentifiers \ 0 \ Paths \ (a4eb1ded-a858-4081-a9e3-4679ac0f687f)]
"Description" = "description"
"SaferFlags" = dword: 00000000
"ItemData" = " C: \ Program \ Avira \ AntiVir PersonalEdition Classic \ avnotify.exe "
This REG_SZ string we put into the executable avnotify.exe system restrictions, this system will be blocked by preventing the implementation and then the annoying advertising.
For Avira Antivir Desktop 9
[HKEY_LOCAL_MACHINE \ SOFTWARE \ Microsoft \ Windows \ CurrentVersion \ Run]
"ATICCC" = "\" C: \ \ Program Files \ Avira \ \ AntiVir Desktop \ \ avgnt.exe \ "/ min / ns"
In the new version need you will need  to write / ns nosplash instead of
[HKEY_LOCAL_MACHINE \ SOFTWARE \ Policies \ Microsoft \ Windows \ Safer \ CodeIdentifiers \ 0 \ Paths \ (a4eb1ded-a858-4081-a9e3-4679ac0f687f)]
"Description" = "description"
"SaferFlags" = dword: 00000000
"ItemData" = " C: \ \ Program Files \ Avira \ \ AntiVir Desktop \ \ avnotify.exe "
Manual Method Of Group Policy
1. Go to Start >> Run
2. type gpedit.msc and clock ok
3. Navigate through User Configuration > Administrative Templates > System
4. Double click "Don’t run specified Windows applications”.
5. Enable it and click show.
6. Add avnotify.exe
7. click ok to close all opened windows
8. Restart