$createdPlan = require 'CreatePlan.php';
use PayPal\Api\Patch;
use PayPal\Api\PatchRequest;
use PayPal\Api\Plan;
try {
    $patch = new Patch();
    $paymentDefinitions = $createdPlan->getPaymentDefinitions();
    $paymentDefinitionId = $paymentDefinitions[0]->getId();
    $patch->setOp('replace')
        ->setPath('/payment-definitions/' . $paymentDefinitionId)
        ->setValue(json_decode(
            '{
                    "name": "Updated Payment Definition",
                    "frequency": "Day",
                    "amount": {
                        "currency": "USD",
                        "value": "50"
                    }
            }'
        ));
    $patchRequest = new PatchRequest();
    $patchRequest->addPatch($patch);
    $createdPlan->update($patchRequest, $apiContext);
    $plan = Plan::get($createdPlan->getId(), $apiContext);
} catch (Exception $ex) {
Update a plan
This sample code demonstrate how you can update a billing plan, as documented here at: https://developer.paypal.com/webapps/developer/docs/api/#update-a-plan API used: /v1/payments/billing-plans/