Login to the Plesk Panel. Then, from the hosting servers menu that located in the left, select DOMAINS.
In Customer Panel select Manage for the relevant domain that you want to choose.
At column on the right, select APPLICATION.
Then, choose the WordPress Version that you need to use and click Install. If already install and need to update the latest version, just click Install button.
You will be taken automatically into the CMS management screen once the installation has been compeleted. Plesk will automatically assign a random username and password. In the CMS management screen you may view your login credentials.