How to add quantity pls help me

const addqtyHandler = async id => {

let itemArray = await AsyncStorage.getItem('cartItems');

itemArray = JSON.parse(itemArray);

console.info(itemArray);

console.info(id);

if (itemArray) {

  let array = itemArray;

  for (let index = 0; index < array.length; index++) {

    if (array[index] == id) {

      console.info(array[index]);

      let qty = Items[index].quantity;

      itemArray = qty + 1;

      console.log(itemArray);

    }

  }

  try {

    await AsyncStorage.setItem('cartItems', JSON.stringify(itemArray));

    await AsyncStorage.mergeItem('cartItems',JSON.stringify(itemArray));

    getDataFromDB();

  }
  catch (error) {

    return (error);
  }

}

}

How to add the quantity of my cart and update it in async storage i dont know how to update my quantity of my cart, i have been searching how to do it, i try to do many things