The first way is that you can go in and edit the version number from the .stm / .pts if you save it as a stamp / local save. I'm not sure which bit to edit to what exactly.

You could also use my mod http://tpt.io/:11117 to open it, it can open any save from all past/future versions as long as there is no major change in the saving format (this happens rarely). I make sure to add beta -> release save compatibility too when I can, like right now where it actually is based off beta 86.2