how to convert older mathcad files (.xmcd) into mathcad prime 6.0

You need to install the full current version of Mathcad 15 to make the converter work. You can access the converter either as standalone application or from within the menu in Prime.

If you have a license for Prime, you are also entitled to use Mathcad 15. The very same license file (not the product number) which you got when you installed prime will also work for Mathcad 15.