I do not understand the question... are you trying to open a Premiere Elements 11 project file in Premiere Elements 12?
I am interpreting your question in a different way than John. I think that you are asking how to switch from Premiere Elements 11 to Premiere Elements 12. If that is correct, then the answer would be just purchase the program from Adobe as an online download or purchase the program from an authorized reseller as a purchased download or in boxed packaging.
The important point to remember is that Premiere Elements 11 and Premiere Elements 12 are standalone programs. There is no upgrading in the old Photoshop/Premiere Pro way. Upgrading in the Premiere Elements world means only a price difference if Adobe recognizes you as an existing customer.
It might be to your advantage to look at the Premiere Elements 12 tryout before purchase to make sure that it is compatible with your computer environment and your video editing/exporting needs.
Please do not hesitate to ask if you have further questions and, expecially if I have not interpreted your question correctly.