Hi,
Plesk should update DRweb definition automatically. Check if scheduled task exists.
It contains the following command.
C:\WINDOWS\system32\cmd.exe /c ""C:\Program Files\SWsoft\Plesk\admin\bin\runtask.exe" "--application=\"C:\Program Files\SWsoft\Plesk\DrWeb\drwebupw.exe\"" "--parameters= /GO /ST \"/RPC:\Program Files\SWsoft\Plesk\DrWeb\DrWebUpW.log\" \"/DIR:C:\Program Files\SWsoft\Plesk\DrWeb\\\"""
Also you can just run the file, for drweb update
C:\Program Files\SWsoft\Plesk\DrWeb\drwebupw.exe