something like
cd /home/user/Desktop/
wget http://www2.ati.com/drivers/linux/amd-driver-installer-12-4-x86.x86_64.run
chmod 755 amd-driver-installer-12-4-x86.x86_64.run
chmod +x amd-driver-installer-12-4-x86.x86_64.run
./amd-driver-installer-12-4-x86.x86_64.run
sync
coldreboot
should work. if you need a different version, just find the URL on AMD's site and substitute it.
to uninstall AMD drivers:
/usr/share/ati/fglrx-uninstall.sh --force