const options = {
  method: 'PATCH',
  headers: {
    'x-spree-api-key': '<api-key>',
    Authorization: 'Bearer <token>',
    'Content-Type': 'application/json'
  },
  body: JSON.stringify({variant_id: '<string>', quantity: 2})
};

fetch('http://{defaultHost}/api/v3/store/wishlists/{wishlist_id}/items/{id}', options)
  .then(res => res.json())
  .then(res => console.log(res))
  .catch(err => console.error(err));

curl --request PATCH \
  --url http://{defaultHost}/api/v3/store/wishlists/{wishlist_id}/items/{id} \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --header 'x-spree-api-key: <api-key>' \
  --data '
{
  "variant_id": "<string>",
  "quantity": 2
}
'

{
  "id": "wi_UkLWZg9DAJ",
  "variant_id": "variant_UkLWZg9DAJ",
  "product_id": "prod_UkLWZg9DAJ",
  "wishlist_id": "wl_UkLWZg9DAJ",
  "quantity": 2,
  "variant": {
    "id": "variant_UkLWZg9DAJ",
    "product_id": "prod_UkLWZg9DAJ",
    "sku": "SKU-557",
    "options_text": "",
    "track_inventory": true,
    "media_count": 0,
    "preorder_ships_at": null,
    "thumbnail_url": null,
    "purchasable": true,
    "in_stock": false,
    "backorderable": true,
    "preorder": false,
    "weight": 0,
    "height": null,
    "width": null,
    "depth": null,
    "weight_unit": "lb",
    "dimensions_unit": "in",
    "minimum_order_quantity": 1,
    "order_multiple": 1,
    "purchase_unit": "unit",
    "units_per_carton": null,
    "price": {
      "id": "price_UkLWZg9DAJ",
      "currency": "USD",
      "amount": "19.99",
      "compare_at_amount": null,
      "amount_in_cents": 1999,
      "compare_at_amount_in_cents": null,
      "display_amount": "$19.99",
      "display_compare_at_amount": null,
      "price_list_id": null
    },
    "original_price": null,
    "seller_id": null,
    "option_values": []
  }
}