Selwyn chipped in with a brace before Mark Korn and Johnny Goldstone wrapped up the win for the early-season pacesetters.
Leeds boss Daniel Kristall said: “Rob has certainly given me food for thought.
"He has been on the fringes season but his introduction turned this game on its head. We were heading for a defeat without him.
"This was a real test of character. We fell behind for the first time this season but we kept our heads and earned our rewards."