Log onto your Plesk account.
On your Dashboard, click 'Wordpress' on the left-hand side of the page.
From here, click 'Updates' near the top of the screen, below the header.
This will pop out half a page from the left-hand side.
You can check for any new updates for Wordpress by click 'Check Updates' near the top of the page. To update your Wordpress, click on 'Update' on the left, near the top of the page.