let input = Storefront.CustomerUpdateInput.create(
  firstName: .value("John")
)

let mutation = Storefront.buildMutation { $0
    .customerUpdate(customerAccessToken: "<customer access token>", customer: input) { $0
        .customer { $0
            .firstName()
        }
        .userErrors { $0
            .field()
            .message()
        }
    }
}

let task = client.mutateGraphWith(mutation) { response, error in
    if let mutation = response?.customerUpdate, let customer = mutation.customer {
        print("[INFO] Update Customer success: \(customer)")
    } else {
        print("[ERROR] Update Customer failed: \(error)")
    }
}
task.resume()