First off, you need a backslash between ".." and "local", and you need to disable string escaping to ghet teh backslashes in there at all:
string el = @"..\Local\App folder\app folder\app.exe";
Then you need to use the Process class:
Process.Start(path);
But you'd be better off using the
SpecialFolder.LocalApplicationData
enum value instead:
Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData)