simple answer you cant
a app created with a version of Mendix can only be opened in this version of Mendix
if you open it in a newer one it needs to be converted to the newer version opening it with the older version will not work
(think of it like this every version has a specific set of features how should an older version know what to do with parts of the project it does not have the features for)
there are some tools to change the version number of an mpr file however i would strongly discurage using them as they will most likely break functionality