CADWorxP&ID > Process Lines > Decrement 
Line Number toolbar: Line Number Decrement
P&ID menu: Accessory > Line Numbers > Decrement
Command line: PIDNUMBER-
Decreases the line number count value by one. For example, if the current value is
0002, Increment 
decreases it to 0001.
Line number count must be properly set. For more information, see Count.