Lots of SCi fi/fantasy universes have vampires. Which was the first of these to have a cure that could turn someone back into a human after they were infected? How did it work?
Depends on what counts as "cured". The following is cribbed from my answer to the question What was the first story to depict a vampire who does not feed on humans?.
1956: "She Only Goes Out at Night . . ." by William Tenn, first published in Fantastic Universe, October 1956, available at the Internet Archive, is about a young lady vampire named Tatiana
Latianu . . .
He came back about eleven-thirty, looking as old as his father. I was right, all right. When he'd wakened Tatiana and asked her straight, she'd broken down and wept a couple of buckets-full. Yes, she was a vampire, but she'd only got the urge a couple of months ago. She'd fought it until her mind began to crack. Then she'd found that she could make herself invisible, when the craving hit her. She'd only touched kids, because she was afraid of grown-ups—they might wake up and be able to catch her. But she'd kind of worked on a lot of kids at one time, so that no one kid would lose too much blood. Only the craving had been getting stronger . . .
. . . and the country doctor who cures her:
The only thing none of us counted on was Doc. Not enough, that is.
Once he'd been introduced to Tatiana and heard her story, his shoulders straightened and the lights came back on in his eyes. The sick children would be all right now. That was most important. And as for Tatiana—
"Nonsense," he told her. "Vampirism might have been an incurable disease in the fifteenth century, but I'm sure it can be handled in the twentieth. First, this nocturnal living points to a possible allergy involving sunlight and perhaps a touch of photophobia. You'll wear tinted glasses for a bit, my girl, and we'll see what we can do with hormone injections. The need for consuming blood, however, presents a somewhat greater problem."
But he solved it:
They make blood in a dehydrated, crystalline form these days. So every night before Mrs. Steven Judd goes to sleep, she shakes some powder into a tall glass of water, drops in an ice-cube or two and has her daily blood toddy. Far as I know, she and her husband are living happily ever after.
It's possible the first cure for vampirism appeared in the soap opera Dark Shadows.
Episode 467, broadcast on April 9, 1968 has a doctor reveal to a vampire, Barnabas Collins, that Barnabas has been cured. The method is underwhelming in that the doctor isn't even sure how he did it, just that he used experimental drugs and that they noted which drugs so it can be reproduced.
It does seem a bit hard to swallow that the first cure was as late as 1968, but it seems that in most previous stories where a cure was even sought, it ended in failure.
We see this search and failure in the penny dreadful Varney the Vampire (1845–1847), the film Dracula's Daughter (1936), and the film House of Dracula (1945).
It's hard to be certain without reviewing the entirety of vampire fiction, but this seems to be the conclusion drawn by https://www.vampires.com/search-for-a-cure/
I can't really say whether a random website is authoritative on the matter, but they've at least reviewed a large breadth of vampire media.
In addition I've looked through The Vampire in Science Fiction Film and Literature in Google Books for instances of the word "cure" with the similar results. Again, I can't really vouch that this is a particularly scholarly and properly researched source.