There are several that spring to mind
1. The Allied bombing of the beach defenses is far more successful and manages to eliminate the barricades on the beach exits from Omaha. This will mostly reduce losses rahter than change the war, but it still a substansive difference.
2. Somebody actually asks a former resident or investigates the terrain in the boccage country and plans accordingly. This will allow the Allies to reach France with tanks already modified to cut through the hedge roots rather than having to suffer losses and serious delays until a fairly clever sargeant assigned to a tank units creates one using steel from the beach defenses.
3. The strategic bomber force is kept in a support role longer, perhaps two extra weeks.
4. More heavy gun ships (14" and larger) are dedicated to the initial gun line. The German High Command stated during post war interrogation that the most puzzling (and frightening) thing about the Allied invasion was how to deal with the sea based fire support. The Heer commanders couldn't understand why the Allies, with all their battleships, didn't have more there to pound the defenses and troops moving up to interdict the invasion force.
5. Better weather in the two months following the invasion. More clear days = way more destroyed Gernam columns
6. Better training for the C-47 pilots. This will greatly improve the results of the airborne landings AND will reduce losses in the Airborne Army, this will pay dividends for the rest of the war.
Now for a couple that will probably cause some disagreement-
7. Bring in USMS ground controllers and Marine/Navy fighter bombers to support the landings. These men were the experts on close air support and should have been used.
8. Either give the object of taking Caen to the Americans or replace Montgomery with a different British commander (my personal first choice would be Field Marshal Blamey, followed by Wavell). Blamey is my choice because his presence also would mean...
9. At least one Australian division as part of the Overlord TOE. The Australians, along with the Canadians, were the best troops the Commonwealth had to offer. The 6th or 7th Australian would have been a major asset in France.
10. Reduce forces in Italy & redeploy them to France. The place was dead end. Keek enough men to force the Heer to maintain its defensive deployment send the rest to France.