It is not recommended to change the default Plesk port because it can break Plesk integration with other programs (like Plesk Expand or DrWeb).
Linux
To change the port Plesk listens on you will need to edit /usr/local/psa/admin/conf/httpsd.conf file and change the following directives to list the port you want Plesk to listen on:
Listen 8443
Port 8443
<VirtualHost *:8443>
You will need to restart Plesk afterwards and update the hardware object in the SoftLayer portal with the new port (Hardware > Click on Server name > Password Update / History)
Plesk cannot listen on any of the ports used for common services (21, 22, 23, 25, 53, 80, 110, 443 etc..).
Windows
Most Plesk installs will be using Apache by default. You will want to edit the Apache configuration file C:\Program Files\SWsoft\Plesk\admin\conf\httpd.conf
1.) Change the following line to list the port number you want Plesk to listen on
Listen 8443
2.) Restart Plesk Control Panel
You will need to restart Plesk afterwards and update the hardware object in the SoftLayer portal with the new port (Hardware > Click on Server name > Password Update / History)