You can edit replays!
My preferred method is:
1. Wait till before where you want to edit
2. Hit the P key
3. Hold Shift and hit P until you're at the exact frame you want to edit from
4. Press the E key, the game interface should change to where you can now play from that point in the replay
5. When you finish, save the replay under a) the same name, or b) a different name if you want a whole different replay file
Done