If you leave it alone, it will be replaced by some other font. Have you already checked to see whether that result is a problem for you? If not, just leave it.
If you must change it, then you will have to find all the places where it occurs. To avoid this in the future, you can set up distinct styles for the different fonts you are using in your book.
PS It is quite possible to use fonts that are not present in iOS via automatic embedding, but they have to be .ttf or .otf. Unfortunately Lucida Grande is .ttc format.