* Naves is a word that refers to the central part of a church or to ships.
* Changing two vowels means substituting two vowels with different vowels.
Let's try some possibilities:
* "Naves" to "Nives": This could refer to something sharp or pointed, like knives.
* "Naves" to "Noves": This might be a word related to the number nine (like "novenas" - nine days of prayer).
The best solution depends on the context of the riddle. Is there any other information? For example, is there a hint about the genre of the fiction book?