Edgware took a 12th minute when Steve Newing superbly lobbed Paul O'Reilly but United levelled on 17 when Mick Swaysland latched onto Marc Liburd's through ball and beat a defender before firing his shot under advancing keeper Andy Carter's body.
Edgware went 2-1 up on 28 when O'Reilly and defender Mark Newson seemed to leave an aimless ball to each other and Peter Sutton nipped in to score into an empty net.
Only shoddy Edgware finishing and good goalkeeping by O'Reilly meant that United weren't as good as out of the cup by half time but they were back in it on 43 when Peter Scott set up Swaysland to fire the equaliser from outside the box.
Ducks went 3-2 up on 55 when Justin Skinner broke out and fed Kevin Davies down the left who crossed for Swaysland to slide the ball home to complete his hat-trick.
That knocked the stuffing out of Edgware and it was 4-2 on 68 when Swaysland made the most of hapless defender Shamus Finnerty's mistake. Edgware's misery was complete on 89 when Davies headed in Jim Meara's curling cross to make it 5-2.