Replays are stored in a folder that is automatically C:/Users/AppData/LocalLow/HydroSim/HydroSim/Replays. If you can't find it, just search your C:/ drive for "Replays".
Replays automatically name themselves after the date and time they started. You can rename them when you look at them in their folder. You can also delete them here if you think they are taking up too much space.