how to order a replacement airpod pro?

If your Airpod Pro is crackling, you may not need a replacement

Please follow the steps at If your AirPods Pro make crackling or static sounds – Apple Support for troubleshooting.

Hello! Currently Apple has recalled the AirPods Pros for this issue. To get a free replacement you can either: Go to your local Apple Store, Call Apple (You can google their phone number for your country/region) or you can Message Apple on the Apple Support App or iMessage.

I hope this answered your question :D