If you want to request a refund:
You should have gotten your license code sent to your email. If you cannot find it, you can restore your license code in two ways:
Family sharing is supported by our app.
To receive more information about family purchases, please check the website https://support.apple.com/en-us/HT201060 and contact Apple directly.
Some Windows users are having trouble uninstalling old version of FYM, called STAMP. You may see the error saying something abou NSIS uninstalled being wrong.
We've made a short video how to remove STAMP manually from the system: