funced [OPTIONS] NAME
If the $VISUAL environment variable is set, it will be used as the program to edit the function. If $VISUAL is unset but $EDITOR is set, that will be used. Otherwise, a built-in editor will be used. Note that to enter a literal newline using the built-in editor you should press Alt+Enter. Pressing Enter signals that you are done editing the function. This does not apply to an external editor like emacs or vim.
If there is no function called NAME a new function will be created with the specified name
>_ funced fish_prompt
This will open up your editor, allowing you to modify the function. When you're done, save and quit. Fish will reload the function, so you should see the changes right away.
When you're done, use:
>_ funcsave fish_prompt
For more, see funcsave.