He died in 1778 in Paris, so by then the Declaration of Independence and the Battle of Saratoga had already occurred. I'm interested to know his response to the conflict at the time.
Voltaire never wrote about the American Revolution.
That being said, he was very interested in America and the English colonies there. At his death, he owned 13 books on the American continent - mostly anthropological/philosophical accounts of voyages made by the English and French. He agreed with the Abbot Raynal, whose 7-volume book "Political and Philosophical History" claims that the American continent is unsuitable for European colonization and that the natives are a naturally inferior race. However, Voltaire and Raynal also condemned the practice of colonization as it pertains to slavery and subjugation, both physical and political.
However, Voltaire was optimistic about the colonies - even if he disagreed with their establishment. In the fourth letter of his "Letters on the English," we writes that the Quakers' ability to thrive peacefully in Pennsylvania is a testament to the religious tolerance of the colonies, and is sharply contrasted with the intolerance of the English mainland, where state religion "will swallow up all the rest."
On Voltaire's death bed, nearly 50 years AFTER the publication of the "Letters on the English," the Abbot Veri attests in his "Diary" that Voltaire was feeling more optimistic about the prospect of freedom of religion in England, but he never mentions the American colonies.
Up until the point that the War of Independence, the French thought about the colonies in terms of England. We're a little biased now when we read pre-1790 texts because we know the outcome of the war, and we want to project American Revolutionary ideas into the pre-revolutionary colonial period. For Voltaire, the religious tolerance in Pennsylvania was an ENGLISH trait, not an American one.
Voltaire likely died never believing that the colonies would become independent.
Sources you can find online! (among others - this is a big field after all):
Abbot Veri. "Journal." Paris: Tallandier, 1928
Voltaire. "Lettres philosophiques" Paris, 1734.