Your code will not change the default printer outside your application. If you want to change the default printer, you will need to make use of interop. Following method in particular:
[DllImport("winspool.drv", CharSet = CharSet.Auto, SetLastError = true)]
public static extern bool SetDefaultPrinter(string Name);
<DllImport("winspool.drv", CharSet:=CharSet.Auto, SetLastError:=True)>
Public Shared Function SetDefaultPrinter(Name As String) As Boolean
You can then call this function with the name of printer. Note that the printer you are specifying should already be installed on the computer.