If you are using Prime now, on a subscription license, you are entitled to install, license (using the very same license file that you used for licensing Prime) and use Mathcad 15 until the end of this year.
If you do not have a license for Prime, nor Mathcad you can obtain a trial license (for both). See here for more information.
You should realise that if Mathcad 15 suits you, and you want to use it for a longer time, you have to buy a special license that will run out in a couple of years, after which it stops and cannot be revived.See here. Mathcad (14,15) and Prime (1...6) contain the third-party symbolic engine MuPad; PTC have decided to discontinue that symbolic engine, their agreement with the supplier of MuPad is stopped.