In order to select different IronPython version to execute a particular script follow the below steps:
1)Browse to "Scripts" tab under "File > Document Properties"
2)Select the script for which you want to change the IronPython version and click on "Edit" button.
3)A new window "Edit Script" will open up, on the right hand corner you will have a dropdown "IronPython version:" to choose the different versions of IronPython,
Note: The latest supported version of IronPython is selected by default for new scripts.