Search drivers for touchpad driver for your model AND your OS (Windows 7, XP, etc.)
Usually they come in .ZIP or .EXE form. If .ZIP, usually they contain a setup.exe file that will run the installation for you. If .EXE, just run that bad boy and it should do it for you.
The above only works if the file you downloaded is newer than the one already installed. In the case of the driver being the same, and still needing to re-install…you will have to do the following:
Right click on My Computer
Go to Manage
Go to Device Manager
Find the touch pad (probably under Mice and other pointing devices)
Right click on the touch pad device and do ‘uninstall’. If it gives you the option to remove the existing drivers, do that too.