There is two way to install linux ati driver, automatic and package generation. Therefore, the uninstallation process depend on the way we install it.
% cd /usr/share/ati
# must be done as a root
% sh ./fglrx-uninstall.sh
2. Package generation
If you generate the package, then you install it using the package manager available in your compie, so just uninstall it using that. Simple.
Don't forget to uninstall it before install a new version!