To change Terminal Service/RDP port please follow these steps-

*****You are about to make changes in server’s registry so any incorrect changes will result in severe problem*****

1. Open registry editor by running this command in run – regedit
2. Locate for HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Terminal Server\WinStations\RDP-Tcp
3. Locate the ‘PortNumber’ subkey and find the value of d3d (in hexadecimal), change the ‘Base’ radio button to ‘Decimal’ and modify the port no. from 3389 to new one.

Note: If your server is behind hardware/software firewall please make sure to add this port in allow list so that you can establish connection from outside.