The Reverse One Pipe command is used to swap the starting and ending points of a selected pipe.
To reverse the direction of a single pipe, go to
Ribbon: DM Plumbing->Pipes-> 
Reverse One Pipe
Pulldown Menu: DM Plumbing->Pipes->Reverse One Pipe
You will be prompted to specify the pipe for which the direction is to be reversed.
Select pipe to reverse:
The selected pipe will be reversed.