If you're looking for the Chinese character "fu" (福), which means "good fortune" or "happiness," it's spelled "fu" directly.
If you're looking for the Japanese word "fu" (風), which means "wind," it's spelled "fu" as well.
If you're looking for the informal, slang term "fu" used as an insult or expression of anger, it's spelled "fu" as well.
It's important to consider the context and what you're trying to communicate when choosing how to spell "fu."