Thoughout, this guide you'll see how to set your rDNS for your dedicated server directly from your client area.
1) First you would need to login to your Manager and go to My Services. Click on the concerned server you would like to change the rDNS for.
2) From there you would just need to click on the rDNS tab and input your domain / subdomain you would like to set as your rDNS / PTR Records and click on Change.
If successful, you'll receive a confirmation. Please note that you would need to have an existing DNS zone for it to work properly.