If you've followed the instructions of installing Hadoop on your system from some online source and you've made sure to set the paths for your hadoop environment, it's quite simple
This will list all the files present in that folder and you'll find yarn-site.xml.
It's a common occurrence to not have a proper track of things during a Hadoop installation because when you use multiple online resources, you can have conflicts and inconsistencies in your paths and installation locations. It's highly recommended to take a proper big data course to overcome these problems. That way, you can get all your instructions from one source with no conflicts.