You can't - you need it's Process to access it.
Think about it: if I run two copies of the same program, which one should you close?
You can use
Process.GetProcesses[
^] to get all the currently running processes, then
Process.Kill[
^] to kill a specific process.
Each Process object has properties you may be able to use to identify thr one you want:
Process Class (System.Diagnostics)[
^] including the ProcessName and MainWindowTitle