It's simply not possible.
Compare opening a ms word 2016 file with Ms word 2
The main sad difference being that Word 2016 still is able to save files in older formats which then can be opened by Word 2.
Prime 5 is not even able to save in Prime 4 format, let alone in real Mathcad format (version 15 or below).
You can use Mathcad Prime 5.0 full version for 30 days (then it goes into Mathcad Express version): https://www.ptc.com/en/products/mathcad-express-free-download