Here's how to delete them manually.
Renpy Engine has 2 separate locations which it puts the save folders in
The 1st one is in:
C:/Users/[you]/AppData/Roaming/itch/apps/[game]/[game again]/game/saves
The 2nd one in:
C:/Users/[you]/AppData/Roaming/RenPy/[game]
Renpy engine names the save files after the pages and slots respectively. So the 1st slot on the first page would start with 1-1.....save (i forgot the rest of the naming scheme). Delete the ones you wish from both locations, and have fun!