I want to install MYM12.

All you need to do is run the install program on the CD and go through the install. This will create a new folder on your machine where MYM12 will be installed. The install program will not touch your existing version or data. Once the program is installed, you can tell the install program where the datasets exist for the prior version of MYM. The install program will copy these folders under the new MYM12 folder and then upgrade/convert them to the MYM12 format. Then, all you need to do is change your Desktop icon to run MYM12 instead of MYM11 using the utility included on the CD to prevent two copies of MYM from being opened at the same time in Windows.