How to change permissions of file/folder in PLESK

1. Open up your Plesk Control Panel.
2. On the Domains tab section click on [domainname.com] .
3. Click on the Websites & Domains tab.
4. Click on the File Manager Icon.
5. Click Root Directory in the menu.
6. Scroll over httpdocs and click the drop-down arrow
7. Select Change Permissions

   8. Once the Permissions page has loaded, check to make sure that any applicable users have full control on the files. 
   9. To do this select the user (ex. Plesk IIS WP User) and check the Full Control permissions box for the account in the right sidebar.

 
For further detail information you can read at this link https://docs.plesk.com/en-US/onyx/customer-guide/websites-and-domains/website-content/setting-file-and-directory-access-permissions.70738/