Deleting individual saves you can do by simply hovering over the save slot and hitting delete if you're on a device with a keyboard.
If you want to wipe all your progress you'll have to delete save directory. Renpy doesn't save it in the game folder itself, which makes migration between versions quite easy.
On windows they are located in C:Users/YourName/AppData/Roaming/RenPy/GameName
For different platforms you can find the paths where the save directory is located over here: https://www.renpy.org/doc/html/config.html#var-config.save_directory
You should find a folder called "ConwayGame-(SomeNumber)" and that's where all the saves are located, along with the persistent data (the stuff that unlocks the gallery).
Deleting that should get rid of ALL your progress if you want to start over entirely.