There's a tutorial on it. But I'll save you some time:
Right click on a replay file in Windows Explorer - Open with... - Notepad. You will see a lot of numbers. Near the top, where it says NEWGAME: and then some numbers, the second number should say 500 (default number of match frames). Change it to the desired number of frames, save it, then open the replay with Toribash.
Sorted.