What if the 20th of July 1944 Hitler has been "terminated"

[Jolt]

How they would have convinced the world to stop the war? What they would have to do?

Well, the Allies only agreed on unconditional surrender in 1945, but that doesn't mean they didn't think that was the only or proper way to end the war.

In the end of July, the Allies were still struggling to build a bridgehead in Normandy, while the Soviets were already pushing through the Baltic countries, Bielorussia and Ukraine. Phreaps the Allies wouldn't have pushed for a unconditional surrender, but Stalin had already smelled blood in the water and bent on bringing the entire Eastern Europe under his bootheels, I doubt he would have settled for a conditional surrender. Further, most of the conspirators and the future cabinet would have far greater sympathies for the Western Allies and make a transition to a Conservative Democracy, then they would to anything similar to Communism (Which was the archenemy of Fascism/Nazism).
Also taking into account it would take a few weeks before control was cemented by the conspirators and any counter-coups thrwarted, before the conspirators could actually press for an armistice.

Following the debate on the Treaty of Versailles and how the Allies were determined in 1944 to show to the Germans they had lost the war, would this assassination change the German end?
Or would this been seen by the Allies as another trick to avoid a defeat then to claim few years after that Germany wasn’t defeated?

It is true Roosevelt that Roosevelt and Morgenthau were by then already planning to utterly destroy any semblance of German power after the war, the coup and assassination of Hitler might have changed his minds as to how zealous would they be to persue Germany into its hinterlands. I suppose it would all depend on what measures the Conspirators would take to deal with the more radical elements of the regime as a whole.

What about the extermination camps?

That would be a tricky matter. I am not aware of how much the conspirators knew of the extent of the holocaust, but the sheer horror of such a program was the creation of Hitler and more radical elements within the Nazi Hierarchy. Without a doubt, the Conspirators would pull the plug on that as soon as they could, along with other forced labour measures occurring in Germany.

Would Stalin be convinced to stop his offensive?

As I said, that would be highly unlikely.

Stalin had placed great trust in Hitler and cherished the idea of a military alliance up until the very moment when he was attacked (Stalin's suspicions and dismissal of Allied reports that Germans were massing troops on his borders as attempts to alienate both countries apart; along with his orders not to counter-mobilize before the invasion, and not to counter-attack once the Germans began attacking), he felt betrayed, further, his country suffered a great deal from the invasion and now he was winning and on the verge of occupying large tracts of Europe, he was going to stop? I doubt it.

Would Churchill and Roosevelt convinced to halt the D-Day offensive?

They might be inclined to it, but operations would proceed. In the end, I believe Stalin's firm refusal to accept anything less than unconditional surrender would force both Western Allies to continue their operations in Europe, which also settled well with their desire to punish Germany. Regardless if the Western Allies were to accept a surrender without their Eastern Ally's approval, German forces in France, Belgium, Luxembourg, the Netherlands and Italy would have to surrender and be disarmed, and Germany would have to fend off for itself against the Soviets.

In the internal situation what and who to arrest? The SS are an army in the Army. What about Himmler, Goering and the hard Nazi?

In that event, all of the Nazi's high cadre who didn't join the conspirators would be put under arrest. Starting with the SS, which was part of the plot from the beginning. They were the only ones who had the capability to resist and attack the coup once it was well settled.

More I think about it, less I give a chance of success to the generals to stop the war at this point…

Yeah, pretty much. Provided they were completely successful internally, they would then petition the Allies for an Armistice, along with all the conditions of post-war transition to a Democracy, pledges to curb the Nazi regime, restoring sovereignty to the occupied countries (quite possibly including Austria), closing down the concentration camps, trialling the high Nazi elites (Though I doubt much of the military would be effected by it. The conspirators were still very much dependent on the army loyalty to assert control over the country), etc. Churchill and Roosevelt, would start having second thoughts and might have been inclined to take the deal, but only with the agreement of Stalin, who, fiercely rejecting any such agreement would insist on unconditional surrender and occupation under the victorious Allies' terms. That would end the openness by the Western Allies to accept peace.

Realizing they had no choice but to continue to fight to defend Germany from total occupation, they would do so, albeit with weaker decisiveness then Hitler. Once the Germans were pushed out of Poland and France, I believe then the Conspirators would see that all was really lost, and after making a token defence to save some face and dignity, they would fold before the facts and accept unconditional surrender.

The result would be the Soviets putting the whole lot to trial along with the already arrested German elite. They would probably get a few dozens years each with crimes against the peace or something, and the whole path would not deviate that much from our history. A good more deal of Germans would probably have survived though, especially in the Western theaters.