So, you've just dabbled with the new update and tried a few online races, and a some nice person decides to wipe you and as many others out as they can out at Les Combes, so with bits of your car strewn all over the place (the damage model was quite good!), you retire to look at the replay to see who it was. However, while I found a monitor screen, I couldn't do much apart from spectate the ongoing race, as it had no rewind button or slider, and all you can do is exit, and I never saw a button anywhere along the way from retiring to leaving the track to save a replay, so how do you do it? I can do this in rFactor2, and that has a lousy UI.
You need to wait for the race to complete and get to the results screen. The "save replay" button should be visible on the right hand side of the screen. Save the replay, then exit the lobby and view the replay through the normal menu. I've done it many times.