For mild cases of nightmares, I recommend something called image rehearsal therapy. It works by desensitizing the nightmare so it’s less scary.
During the day, "write out your nightmare, remembering the early details. But near the end of the story, during the most frightening part, create a non-scary ending."
Now you have a story that starts out like your nightmare, but has a safe ending. Visualize this story a few times a day. When the story appears in your nightmare, the ending should change to be more like the one you’ve created.
This may extinguish the nightmare over time.