Although in theory you should be able to have multiple versions of BikeCAD installed on your computer at once, lately, it seems that things work better if you uninstall older versions as well as any copies of the free version if you've installed that. The reason for this is that if I install the latest version of BikeCAD which at this point is version 18, the program is meant to be installed in a directory in your user directory called AppData\Local\BikeCAD Pro 18.0 . This is where it will go on a clean computer, but if older versions exist, inexplicably the new version of the program will get installed in the folder for the older version and will give you errors when you try to run it.
To avoid this, I'll find the older version of BikeCAD among my apps. I'll right click on the app and choose Open file location. This will bring up the Windows File Explorer with what is in my case a link to the free version of BikeCAD. This is actually just a shortcut, so I'll right click on this and choose Open file location once more. This will take me to where the program is actually stored. I'll scroll up to the app folder and notice a subfolder here called BikeCAD_17.5_configuration . I'll copy this folder to my desktop just in case I want to copy any of my old configuration files into the new version. I'll go back to the app listing in the start menu and right click and choose Uninstall. Once the old version has been uninstalled, I'll want to return to the AppData\Local\ folder and delete the BikeCAD Free 17.5 folder from there. This final step will ensure that when I finally go to install BikeCAD Pro version 18.0, the program will get installed in a new folder called BikeCAD Pro 18.0.