In Plesk you can change your file permissions as follows:
1.Login to Plesk
2.Click File Manager
3.From the list of files displayed, click the More icon on your extreme right as shown below. An option dropdown menu will appear with Change Permissions option
4. On the page that appears, select the Permissions needed then click Save. Note that for some files, permissions cannot be changed.
Video
Below is a short video on the same procedure