I am using following code, but nothing was changed after trying.
if (!is_null($cpcAmount)) {
$bid = new \Google\AdsApi\AdWords\v201809\cm\CpcBid();
$money = new \Google\AdsApi\AdWords\v201809\cm\Money();
$money->setMicroAmount($cpcAmount * 1000000);
$bid->setBid($money);
$biddingStrategyConfiguration = new \Google\AdsApi\AdWords\v201809\cm\BiddingStrategyConfiguration();
$biddingStrategyConfiguration->setBids([$bid]);
$adGroup->setBiddingStrategyConfiguration($biddingStrategyConfiguration);
}
// Create ad group operation and add it to the list.
$operation = new AdGroupOperation();
$operation->setOperand($adGroup);
$operation->setOperator(Operator::SET);
$operations[] = $operation;
// Update the ad group on the server.
return $adGroupService->mutate($operations);