Simply go into the app where your file is, and look for an "Open" or "Share" button. The buttons usually look something like this:
Next, you will see a screen like this, where you can choose the Converter app. (Note, you might need to tap "More ..." if you don't see the Converter app in the list)
Your file will then open in the Converter app, and you can choose what file-type to convert to.