Not sure which Forest Pack Pro version are you using now (the support for 3ds Max 2015 was added in Forest Pack Lite/Pro 4.2.3
), but I would suggest you to proceed the installation of the latest release - Forest Pack Pro 4.4.1
. There is no need of any special process (the installer will uninstall the older version automatically).
The installer can be downloaded from the User Panel
under Downloads - My Products
. To access it please log in to client site using your user details. The Client login
link you can always find on the top of our website
Hope that helps.