How change the scipr of existing DS thing?

[inferjus4]

What would I have to do to like change the recipe (not add new one) of existing DS item/object or how add new loot that drops when killing existing mob?

[Ultroman]

In order to overwrite an existing recipe, all you have to do is call Recipe() with the first parameter being the name of the prefab whose recipe you want to change. It's the same as when you add a new recipe. It just overwrites the original recipe.

You seem to be asking two very different questions here.

Changing what loot drops from certain mobs/entities is a subjective matter, as the lootdropper-component can be set up in various ways. Some have random drops, some have weighted drops, some have static drops, some have state-based drops etc.. Look at the lootdropper-component. Read the code there. Go to the prefab you want to change, and see how it sets up its lootdropper-component. Return with questions  Search for lootdropper on the modding forums (see my newcomer post for tips on how to search this forum properly)