You need to load order instance either from order entity_id or increment_id. After that, try these code. I haven't tested these code
$payment = $order->getPayment();
$order->getPayment()->importData(['method' => 'yourpaymentmethodcode']);
In my case Firefox was auto-filling the "PayPal Express Checkout" configuration with Magento login credentials. Click "Configure" next to "PayPal Express Checkout" and remove the auto-filled credentials.