With Plesk 6 and later versions, there is no concept of the Primary IP address for the server. From the control panel's point of view, all IP addresses are equal. The only difference between the main IP address and aliases is that the main IP address cannot be deleted from the control panel. In Plesk interface primary IP address on the network interface will be marked with bold text.
Attempt to remove it in Plesk 10 and 11 will display an error, for example:
Error: Cannot remove the IP address 192.168.1.10 because it is the primary IP address of a network interface.
To change or remove the main IP address, you need to first remove this address from all IP pools in Plesk, as well as all records mentioning this IP from DNS zones of all domains.
Then stop Plesk Panel and manually change the IP address on the server from the backend as root.
Be careful: if you set the wrong network parameters, the server may become unavailable.
Then start Plesk again and restore the list of IP addresses through SERVER -> IP Addresses -> clicking on the Re-read button. If the IP still exists in the Plesk interface, remove it directly from the Plesk interface or delete it from the "psa" database with a query like the following:
mysql> delete from IP_Addresses where ip_address='YOUR-IP';
Note: If you are using Plesk v9.x, please see this article regarding main Plesk IP removal #5967