Change read/write permission of website on Plesk Windows server

+ Log in control panel, go to Subscription -> Files tab -> browse to directory or file you want to change permission.
+ Hover mouse over the file/directory to show a small down-arrow button next to modified date. 
+ Click on arrow and select Change Permissions.
+ Select Application pool group (IWPG_youraccount) and choose allow "Full Control".
+ Click OK to save changes.

It's recommended to NOT enable full control on website root folder (httpdocs).

If you want to modify a file, enable write permission on that particular file.

If you want to upload files, create a subfolder and enable write permission on the subfolder.