I think there are already posts that explain that on this forum but in short : 1) Make a new sandbox game with the settings you want . 2) go to //users/yourname/Zomboid/saves/sandbox/ Make a backup of your save (in case anything goes wrong). 3) Copy the file map_sand.bin (where the settings are stored) from your new game, over the top of the map_sand.bin of your old game.
It will change the settings of your game to the settings of the new game with your desired settings. It will only change that nothing else.
If you didn't understand try writing "project zomboid how to change sandbox