To change IP address in Plesk, please follow the below steps,
Login to Plesk panel.
Go to Tools & Utilities.
Go to IP Addresses.
Go to IP Address you want to assign.
If you want to make it a private IP, select the exclusive radio button.
For SSL Certificate leave to default if you didn't installed it already.
In Default site, choose your domain name for which you want this IP address.
Click OK to complete.
Note: when changing IP address, your domain will be down for few hours.