The best way is to use a local variable. In your block diagram, right click on the control. Select Create - Local Variable. You can right click on this local variable and make it readable or writeable. When writeable, just wire in a value. You will see the change being made on the front panel as it happens programatically.
You could create a Property Node and write to the Value property to acheive the same effect, but local variables are quicker.