By default the RDP (Remote Desktop Protocol) port uses 3389. But if you feel that the port is not secure enough, you can use another alternative port by changing it from 3389 to 6665, 3335, 6589, or whatever.
How to change it?
Microsoft has provided an official tutorial on how to change the RDP port on your server. For more details, please use this link. If you are not sure what to do, please contact our support team via email firstname.lastname@example.org so we can help you change it.
Make sure that after the new port is changed, you add the port to the server firewall.
Also read: Opening Ports on Windows Server
After everything is okay, you only need to restart the server.