Click the Modify tab on the Toolbar, click Pipe Network, then click the drop down "Modify" tab. Select the last command, "Change Flow Direction". Follow the instructions on the Command line and your labels will update quickly and easily.
It's very important to pay attention to the direction in which you draw your pipes. You should draw them in the direction shown on the Pipe Creation toolbar. (See attached file.) In the direction shown, draw from the uppermost part of the run to the lowest part of the run. In this way, your flow directions will always be correct. However, if you forget to do this, simply follow the paragraph above and you can quickly make the flow direction correct.
Note, though, that drawing the pipes in the correct direction based on the Pipe Creation toolbar, the pipes will have the correct "Start" and "End" notations. There is no way to quickly correct this if drawn improperly... you have to grip edit them and swap ends manually to make it right. So it pays to pay attention to this part of creating pipe runs.
But changing the flow direction is easy, as shown above.
