Hello and welcome
You need to save the replay first. Press the "2" key to save the replay (you will be prompted to name the replay file).
You can select to view a replay by clicking on "replays" on the main screen and then selecting the replay that you want to view.
Once the replay starts running you can hit the spacebar to pause/play. More keys can be found on the LFS Manual
To show/hide the HUD/on-screen info toggle shift+F.
To play around with the camera views etc., use shift+U and use on-screen menu (shift+U menu can be shown/hidden by pressing the spacebar).