When you download the new version select the "Repair" instead of "Install". This will install the new version without having to uninstall the old version first.
QUOTE('AVG install procedures')
Run the AVG Free Edition 7.5 Setup program.
Click "Next >" on the AVG "Welcome!" screen.
Click "Accept" on the "License Agreement on Use of an AVG Free Edition" screen.
Select "Repair installation" on the "Select Setup Type" screen and click "Next >".
Select "Restart the computer now" (pre-selected by the program automatically) and click "OK" on the "Installation Complete!" screen to restart your computer and complete the installation.
http://free.grisoft.com/doc/3/lng/us/tpl/v5