Honest answer to your question: I don't know! Does the M$ Store also offer the Premium version? I ask, because I suspect you can only upgrade your Store version via that route because all Store apps are installed in a default "sandboxed" (and normally "hidden") location on your computer which cannot be accessed by non-store software. So, any attempt to upgrade via the Magix website is likely to fail because the upgrade cannot "see" the already-installed version. You may need to ask M$ Store support about how to do this.