It quite simple operation, but here is one precondition:
Before you go next, make sure you have pointed A-record of your domain to this IP.
Go to VPS hosting page and select VPS where you want to change rDNS:
Click on the "IP addresses" button at the top button panel.
Next, select the ip address where you want to change rDNS and click "Edit" button. Multiple selection also available here.
Enter new rDNS value.
After changing click "OK" button.
rDNS will be automatically changed for this server.
Remember, records in DNS are not changing instantly, it may take from 5 minutes to several hours. Be patient. Do not repeat the same operation twice.