Restart application within itself

If all you want is a restart then you can do the following:
Use GetCurrentProcess() to get a PSEUDO HANDLE to the current process, and duplicate it with DuplicateHandle() in order to get a non-pseudo INHERITABLE handle. When this is done you can use GetModuleFileName(NULL, buf, bufsize) to get the path to the current executable. You should launch the exe with CreateProcess() with bInheritHandles==TRUE parameter so the next instance you start will be able to inherit the duplicated handle so that the launched process will have a valid handle with the same numeric value referencing the original process object in the kernel. When you start the next process with CreateProcess(bInheritHandles==TRUE) you should pass the numeric value of the duplicated process handle to the child process for example as a commandline parameter ("x\y\process.exe --wait_exit 2345325"). After launching the child process you can close the duplicated handle with CloseHandle(), cleanup everything and then quit.
At startup you should handle the "--wait_exit" commandline parameter and if it is present on the command line then you should convert the parameter into an int and then cast into a HANDLE then wait for it with WaitForSingleObject(). When the wait is finished you can close the handle with CloseHandle(). This way your child process waits until the original process is done.

After waiting with WaitForSingleObject() you can add some extra code to ensure that only one instance is running at a time - I usually use a named event object for that.

If you want to replace the executable file during restart then you should do almost the same but instead of starting the same exe, you have to start another helper exe (updater.exe). This way the updater.exe can wait till the original exe terminates, can replace the exe without trouble and then it can launch the new exe.