In order to view replay files (*.rpy) that people post in the forum areas, or that are included with setup files that can be downloaded elsewhere, save the .rpy files into your Documents / iRacing/ replay folder.
Then, from the iRacing UI click Replays
This page will list all of your replays and then click the green Launch button to launch the desired replay:
https://www.youtube.com/watch?v=jSrh1qaPXnc