With a POD of no more than 4 months before June 6, 1944 and without WMDs/weather/landing date changes, your challenge is to make Operation Overlord much, much bloodier for the Allies. You can marginally, but not significantly increase troop strength, and you can swap out Panzer Divisions for Infantry Divisions and the like. You can wrestle tactical flexibility from Hitler, and you can replace field commanders.
At the same time, avoid disasters like the Falaise Pocket for the Germans. While Axis forces are almost certainly going to lose, the idea is to make things difficult whilst hanging onto as much strength as possible for the defense of the Fatherland.