To set only Anonymous Ftp user can make changes in FTP directory do the following task -
STEP - 1
In plesk control panel select tools & settings button - click on custom view settings.
Then select hosting tools tab - click the check box Anonymous Ftp Management - Apply - ok
Step - 2
Go to select subscription - and select your subscription (domain name) - websites & domain -
select FTP Access.
Create a FTP Account and home directory should be "/anon_ftp"
switch on anonymous FTP service for the domain and select the check box allow downloading from the incoming directory