If you're installing PerfectIt 3 or above, you do not need to uninstall it before re-installing - simply download and install the latest version.
If you're re-installing PerfectIt 1 or PerfectIt 2 then you do need to uninstall PerfectIt first.
There are two ways to remove PerfectIt 1:
You can then download the final update of PerfectIt 1 from here and re-install it.
To uninstall PerfectIt 2, close MS Word. Then open Control Panel, then choose "Programs and Features" (Windows 8/7/Vista) or "Add and Remove Programs" (Windows XP/2000). That will show a list of all installed programs. Find PerfectIt 2 in that list and double-click on it to uninstall it. Then download and install the final release. If you see any error messages during either uninstall or install (or if the installation doesn't work), the best thing to do is to restart your computer and try again.