Login to plesk panel.
Select the domain for which you want to delete the logs
4. Select "Log Manager" under "Statistics".
5. To set the "Lines of log files to be displayed ( from the end of the file)" ,
type in the number of lines in the Lines of log file to be displayed input field
6. To view a log file, click on a log file's name in the list under "Log files".
The log file contents will be displayed in a separate Log File Viewer window.
7. Delete log files, select the corresponding checkboxes, and
click Remove Selected.