Tyrell Waite's 31st minute strike had the new-look Pilgrims in front at the break and Liam Adams doubled their advantage with 15 minutes left to play.
The victory was sealed five minutes later when Richard Ford sliced the ball into his own net.
United (first half, 4-2-3-1): George Willis; Taron Hare, Jordan Keane, Jack Cowgill, Brad Beatson; Jamie McGuire, Adam Chapman; James Clifton, 9 Ashley Hemmings, Tyrell Waite; Kabongo Tshimanga.
United (second half, 4-2-3-1): Harry Payne, Joe Burgess, Tom Batchelor, Jan Yeomans, Mason Warren; Harry Vince, Jack Broadhead; Liam Adams, Karl Hawley, Trialist; Kuda Muskwe.
Referee: Chris Reeson.