Cashier expects a token to be created client side using stripe.js/elements or checkout. You then submit this token to your server route where you call:
// grab the token from the request like this
// $stripeToken = request()->input('stripe_token');
$customer->updateCard($stripeToken);
The Stripe API is itself really well put together and easy to use, so you can use it directly, as well:
\Stripe\Stripe::setApiKey("stripe_secret_test_key");
$customer = \Stripe\Customer::retrieve("cus_CFdeffeSfeefe");
$card = $customer->sources->retrieve("card_1sdafjkfsdD32f");
$card->name = "Joseph Jones";
$card->save();