If you already have a version of SketchList 3D installed this window will appear on re-installation. 

 

If you don't want to install the upgrade click on the X in the upper right corner of the form.

 

REMEMBER - moving from Version 3 to Version 4 - the projects will automatically update but are not backward compatible.  If you install V4 and for whatever reason want to go back, make sure you have exported the projects you need in V3 and saved them.  This way you can always import them again.

 

It is a great idea to export important projects when finished with them  It's hard to have too much backup.