What was found
YYZ - FLT 1084 - AFTER TAKEOFF YYZ EN ROUTE TO LGA ON CLIMB-OUT, SMOKE WAS OBSERVED IN COCKPIT WITH BURNING SMELL. SMOKE APPEARED AFTER APU WAS SHUT DOWN IN-FLIGHT. AIRCRAFT RETURNED AND LANDED YYZ WITHOUT INCIDENT. REPLACED APU PER MM. ACCOMPLISHED OPERATIONS CHECK AND LEAK CHECK. SYSTEM GROUND CHECKED, NORMAL OPERATION.