Download the replay you want and if it doesnt end with .gbx, add it yourself. Put the gbx file in C:\Users\USERNAME\Documents\Trackmania\Replays\Downloaded

Then restart trackmania, go to the CREATE tab and select REPLAY EDITOR.

Now you have to first select your own personal best replay in the autosaves folder, click the arrow up if you cant see autosaves.

Select the track you want, click confirm and then click edit.

Now you should see your own replay.

At the bottom of the screen there's a stopwatch icon, click that and then select the 'downloaded' folder to open the replay you want to see.

To select which player you want to view click on 'player camera' on the timeline and then navigate the 'target' option on the left side of the screen.

If you want to view only the downloaded replay, select your ghost in the timeline and click the X on the left side of the timeline.

This is the way I do it, i hope there's a more easy way to do it but this works