Well, you can just install the new one over the old one.
But, to eliminate any type of problems it is recommended that you uninstall the old ones first. Add/remove programs, restart, install new ones.
To be even more anal about it, you can install drivercleaner pro. Restart into safe mode, clean cab and then clean drivers.
I would just do the first one installing ontop of the old ones. If you have problems with the current ones or if you have problems with the new installation, then drivercleaner may help you.
NOTE: installing new drivers over the old drivers is not a common practice, and are only to be done to Nvidia display drivers. All others must be uinstalled before installing new ones as traditionally done.