There are two ways to look at this. Either the French can completely win the 7 years war in Europe, or they can win the overseas portion of it.
France wins in Europe
The fate of nations rests on the fortunes of battle. Nothing would more easily change the tide of the war than reversing one battle. In this case, the battle is Rossbach. In June 1757, France invaded Hannover, won a crushing victory, occupied the capital of Brunswick, and forced the Hannoverians to sign the Convention of Klosterzeven. Since King George of England was also the Elector of Hannover, he was not too keen about this Treaty and looked for the first opportunity to disavow it. He got his wish a few months later, when Frederick routed a French/German force at Rossbach (5 Nov), then the Austrian army at Leuthen (5 Dec). Britain disavowed the convention, Hannover re-entered the war, and the stage was set for the Anglo-Prussian convention, where England bankrolled Fredericks armies. This proved to be the winning combination.
Now lets flip the script: Frederick suffers a crushing defeat at Rossbach. There is no-one to confront the Austrian army at Leuthen, so the Austrians retake Silesia. Now Frederick is facing four major enemy armies (French/German, Austrian, Russian, Swedish) in his home territory. Hanover is still occupied by the French, and since George is from Hanover and cares about that land, he isn't willing to re-enter the war on Prussia's side. Prussia is sacked into oblivion and Frederick surrenders. France uses Hanover as a bargaining chip; to get it back un-razed, England has to give up some Caribbean Islands (Jamaica?) or pay some reparations. France keeps its significant colonial presence in the new world.
France wins in the New World
Britain was saved overseas by an 'Annus Mirablis' in 1759. Note that this is AFTER the war would have theoretically ended in the above scenario.
This war didn't have a clear turning point, but a succession of small changes would have altered the course of the war. Had the Seige of Ft. Carillon dragged on from 1759 to 1760 instead of the French abandoning it, had the British been repulsed at the Battle of the Plains of Abraham, had the British been unable to take Ft. Niagara, the French would have been in a very good position after 3 years of fighting.
The war would have dragged on as a stalemate, but in this case the advantage would have been to the British. They had the naval power to support overseas fighting for years and they still could have worn down the French irregulars and their Indian allies with time. But the French could certainly hope for a stalemate and return to status quo by the end of 1760.
Don't Forget India
In the long run, the most important part of the 7 Years War, which is mostly overlooked, were the encounters in India. In 1757, Robert Clive defeated the pro-French Nawab of Bengal and replaced him with a British puppet. The French failed in a seige of Madras, lost a battle at Wandiwash in 1760, and then lost Pondicherry to the British in 1761. French influence dissapeared from India. Of the three power centers from which the British Raj in India spread (Bombay, Madras, Calcutta), two were won decisively from the French in this war.
One of the biggest alternate history swapperoos of all would be switching the British Raj out for the French Établissements. Had the French won, they would have much more interest in establishing and maintaining a world naval presence. They would not have wanted to sell French Louisiana, even had they lost Canada, and they would have built up a navy to challenge England.