To uninstall PolyBoost you need to have 3ds max started. Perform the uninstall by running the file "Uninstall_PolyBoost.mse" from Maxscript/Run script...This file can be found in 3dsmax/Scripts/PolyBoost/. After uninstalling there will still remain a PolyBoost folder in your local scripts directory of 3ds max. Remove that folder manually. You will also need to remove the file PBPolyBoost.dlx (or PBPolyBoost64.dlx for 64 bit) from the 3dsmax\plugins directory since this file can not be removed automatically while it is loaded in 3ds max.
If you are using the demo of PolyBoost and have purchased the full version, there is no need to uninstall the demo before activating the full version. Simply run the "PolyBoost_Setup.mse" again or press the "Authorize" button in the "Other" section of the main interface which will run the setup. This time enter your serial and authorize.