To upload file through Plesk please follow the following steps:-
1) Login to Plesk control panel.

2) Click on "Domains" and then Click on "Control panel" in front of the domain where you want to upload a files.

3) Click on "Websites and Domains".

4) Click on "File manager".

5) Click on "Add New File" and browse the path of the file from where you want to upload a file.

6) To create new files in the required directory, click Add New File, in the File creation
section specify the file name, select the Use html template check box, if you want
file manager to insert some basic html tags to the new file, and click OK.

NOTE:-You can only upload files from the Plesk file manager. You can not
upload the whole directory from the panel.You need to use FTP
server if you want to upload the whole folder in your directory.