There's an easy method that someone shared on Reddit 7 years ago which is supposedly still working till this day. To save you from visiting Reddit:
while you are in the game alt+tab and go the save folder of your game -now delete every save file you don't want -when you are done, close the game -steam will start syncing and delete the files in the cloud as well.
If this doesn't work, here's the convoluted method.