The test plans are written in YAML format. They are human-readable and can also be utilized directly as release elements by the PLT Operating System. You may modify these in your own coding environment. If you use a Mac, you can also use Sublime Text editor to modify these files and re-save them as a test plan with a new name and/or version number.
We have a guide called the Test Plan Reference for the syntax of the YAML file as well.
Here is an online linter for validating the YAML file: http://www.yamllint.com/