Sorry Ahmed, I'm not sure I understand.  Could you elaborate a bit?

Normally I can center the pivot using c key shortcut when I have enabled Edit on scene .

But what I asking for is how to center the pivot when I am using second scene node after the first one?

You can do it two ways:

a) Instead of selecting the object in the preview, select the desired Scene Node in the Flow Editor,  Then put the focus back to the preview (for e.g. clicking on its header) and then press the C.

b) Select the object in the preview, then press U several times until the desired parent Scene Node is selected, then press the C.