@ema it really is quite simple:
the autosave slot is called Savegame1, the others are named Savegame2,… up to Savegame9
the savegame consists of the .save file and a .png for the thumbnail
So copy/rename any manually saved game and its thumbnail to Savegame1 to put it back in the autosave slot
Or vice versa copy the Savegame1.* files to another one to make a manual save in another slot
Note that you can use the same technique to copy additional save-points in the game without being limited to 9 savegame slots.
No need to worry about the save.dat file (it is recreated on the fly)