The first version of MillWizard used a different file structure that has your license hidden away. That license needs to be removed before you can upgrade to a newer version and install a new license. Using the link below, follow the steps to show hidden folders. 

https://support.microsoft.com/en-us/help/14201/windows-show-hidden-files

Once you can see the hidden files and folders that were previously invisble to you, go to your C: Drive and then Program Files (x86). You should see a subfolder called NFS_Mill. If you don't, check the other Program Files folder. Open NFS_Mill and remove your old license, which should be called WizLic or WizardLic. From there, you should be able to open MillWizard and load your new license encrypted with your PC ID.