1: Open your windows menu (where you can shut your computer down)
2: use the search function for: "notepad"
3: open notepad
4: open your replay (go to toribash then replays)
5: under author you will find the game length, I forgot tbe exact name, but if the replay was 300 frames long it'd say: gamelength: 300
6: edit the amount to 1000 to make it longer for example
7: save at the place you got it from, but watch out that you save it as .rpl
there you have it!