You can just select the right process, like :

                foreach (Process process in Process.GetProcessesByName("notepad"))
                    process.Kill();
						
See Microsoft KB Archive/178893. It is an old Microsoft Knowledge Base article. The C++ code does not help but the concepts are still valid. The important thing is that it says to first try sending a WM_CLOSE message. The Process.CloseMainWindow Method will do that; close the process "cleanly". Kill should be used only if it is not possible to close the process cleanly.    
And if you want to do a more efficient search then the following might help. It is more code for you but it will enumerate the desktop windows and get the process for each.     
[DllImport("user32.dll", SetLastError = true)]  
static extern uint GetWindowThreadProcessId(IntPtr hWnd, out uint lpdwProcessId);  
[DllImport("user32.dll", EntryPoint = "EnumDesktopWindows",  
    ExactSpelling = false, CharSet = CharSet.Auto, SetLastError = true)]  
private static extern bool EnumDesktopWindows(IntPtr hDesktop,  
    EnumDelegate lpEnumCallbackFunction, IntPtr lParam);  
private delegate bool EnumDelegate(IntPtr hWnd, IntPtr lParam);  
public static SortedSet<uint> EnumWindows()  
    var ProcessList = new SortedSet<uint>();  
    EnumDelegate callBackFn = new EnumDelegate(EnumProc);  
    GCHandle handle1 = GCHandle.Alloc(ProcessList);  
    EnumDesktopWindows(IntPtr.Zero, callBackFn, (IntPtr)handle1);  
    return ProcessList;  
public static bool EnumProc(IntPtr hwnd, IntPtr lparam)  
    uint processId = 0;  
    GCHandle handle2 = (GCHandle)lparam;  
    uint threadId = GetWindowThreadProcessId(hwnd, out processId);  
    SortedSet<uint> ProcessList = (handle2.Target as SortedSet<uint>);  
    if (!ProcessList.Contains(processId))  
        ProcessList.Add(processId);  
    return true;  
static void Main(string[] args)  
    SortedSet<uint> ProcessList = EnumWindows();  
    foreach (uint processId in ProcessList)  
        Process ownerProcess = Process.GetProcessById((int)processId);  
        Console.WriteLine(ownerProcess.ProcessName);  
I do not know how the .Net Process.GetProcessesByName method finds the processes but I assume it uses something like in Enumerating All Processes so that even if you request a specific process by name, the method will first get all the processes. So requesting a process by name is not likely to save computer time; it just saves your programming time.    
												
@Sam of Simple Samples   makes some very good points.  Even Windows Task Manager attempts to close a process in an orderly fashion before resorting to TerminateProcess.  See https://devblogs.microsoft.com/oldnewthing/20040722-00/?p=102304.  The Process.Kill method uses TerminateProcess and if called on a process other than your own can result in unanticipated and unexpected problems.  For example, refer to the discussion at crash-during-terminateprocess