To change your PTR (rDNS) record, you would need to determine if your custom nameservers have the authority to make the change. If unaware of the authority, you would reach out to your Hosting Provider to verify.
Method to determine where your PTR record are coming from:
We have a shortlist of known Hosting Providers where PTR changes would need to take place:
If unsure who your Hosting Provider is, use the following article:
Once verified that your custom nameservers do have the authority, then use the following article. The needed steps appear in the documentation.