Theoretically, you shouldn't need to uninstall your old drivers. Just install your new drivers over your old ones. If you really want to uninstall your old drivers before installing newer ones, use Control Panel --> Add/Remove Programs and use the ATI Catalyst Install Manager. If you REALLY want to clean it out:
1) Download and install Driver Cleaner.
2) Download the latest ATI Drivers for your graphic card series and operating system.
3) Use ATI Catalyst Install Manager to remove your current drivers
4) Reboot your system into Safe Mode
5) Run Driver Cleaner to remove any remaining instances of ATI software*
6) Reboot your system, allowing to boot normally and install your newer driver.
7) Reboot your system once more.