The U-value and other analytical properties for windows and doors in Revit are set by default and come from the ASHRAE standard. Sometimes, though, you need different U-values for heating load calculations. So, how do you change these hardcoded values?
You can find these constructions on your computer: C:\Program Files\Autodesk\Revit 2018\en-US\Constructions.xml
Be aware that the “Construction.xml” file is different for different Revit Language Packages. So just make sure you modify the right one.