redux reducer… + 1 I have created a new array and added one. now how do I put it back together and return the new state

so found the index of what I want to change, I created a new array newFavorites and increased a value on one of its keys (sets +1). But I am not sure how to return the new state with the updated key value.

  case ADD_SETS: 
                const favToIncreaseSet = state.favoritedExercises.findIndex(fav => fav.id === action.payload.id);
                const newFavorites = [...state.favoritedExercises];
                let repIncreasedFav = newFavorites[favToIncreaseSet].sets + 1;
               
                
                return { ...state, newFavorites}