You can set reverse DNS for your IPv4/IPv6 address very easy by following the following steps:
1) Login into the VPS Panel and navigate to your VPS
2) Click on the tab named "Network"
3a) To set a IPv4 reverse DNS record, click on "Edit".
3b) To set a IPv6 reverse DNS record, click on your IPv6 subnet.
Changes are being applied instantly. However, due to DNS cache (TTL) it might take some time to before the new reverse DNS record will resolve on your end.